qmp/hmp: crash if client closes socket too early

Qemu crashes on qmp/hmp command if client closes connection before reading the whole response from the socket.

Reproducer:

1. Start arbitrary vm via qemu
2. Send e.g. hmp command 'info mem'
3. Abort before whole response came back


Stack Trace:

Stack trace of thread 6493:
#0  0x0000559902fd2d30 object_get_class (qemu-system-x86_64)
#1  0x0000559903071020 qio_channel_create_watch (qemu-system-x86_64)
#2  0x000055990305f437 qemu_chr_fe_add_watch (qemu-system-x86_64)
#3  0x0000559902f7340d monitor_flush_locked (qemu-system-x86_64)
#4  0x0000559902f7360e monitor_flush_locked (qemu-system-x86_64)
#5  0x0000559902f74342 qmp_send_response (qemu-system-x86_64)
#6  0x0000559902f74409 monitor_qmp_respond (qemu-system-x86_64)
#7  0x0000559902f74bc0 monitor_qmp_bh_dispatcher (qemu-system-x86_64)
#8  0x00005599030c37be aio_bh_call (qemu-system-x86_64)
#9  0x00005599030c6dd0 aio_dispatch (qemu-system-x86_64)
#10 0x00005599030c369e aio_ctx_dispatch (qemu-system-x86_64)
#11 0x00007f5b6d37f417 g_main_context_dispatch (libglib-2.0.so.0)
#12 0x00005599030c5e0a glib_pollfds_poll (qemu-system-x86_64)
#13 0x0000559902dd75df main_loop (qemu-system-x86_64)
#14 0x0000559902c59f49 main (qemu-system-x86_64)
#15 0x00007f5b6bfeab97 __libc_start_main (libc.so.6)
#16 0x0000559902c5d38a _start (qemu-system-x86_64)

#0  0x0000559902fd2d30 in object_get_class (obj=obj@entry=0x0) at ./qom/object.c:909
#1  0x0000559903071020 in qio_channel_create_watch (ioc=0x0, condition=(G_IO_OUT | G_IO_HUP)) at ./io/channel.c:281
        klass = <optimized out>
        __func__ = "qio_channel_create_watch"
        ret = <optimized out>
#2  0x000055990305f437 in qemu_chr_fe_add_watch (be=be@entry=0x559905a7f460, cond=cond@entry=(G_IO_OUT | G_IO_HUP), func=func@entry=0x559902f734b0 <monitor_unblocked>, user_data=user_data@entry=0x559905a7f460) at ./chardev/char-fe.c:367
        s = 0x5599055782c0
        src = <optimized out>
        tag = <optimized out>
        __func__ = "qemu_chr_fe_add_watch"
#3  0x0000559902f7340d in monitor_flush_locked (mon=mon@entry=0x559905a7f460) at ./monitor/monitor.c:140
        rc = 219264
        len = 3865832
        buf = 0x7f5afc00e480 "{\"return\": \"ffff9eb480000000-ffff9eb480099000 ", '0' <repeats 11 times>, "99000 -rw\\r\\nffff9eb480099000-ffff9eb48009b000 ", '0' <repeats 12 times>, "2000 -r-\\r\\nffff9eb48009b000-ffff9eb486800000 0000000006765000 -rw\\r\\nffff9eb4868000"...
#4  0x0000559902f7360e in monitor_flush_locked (mon=0x559905a7f460) at ./monitor/monitor.c:160
        i = 3865830
        c = <optimized out>
#5  0x0000559902f7360e in monitor_puts (mon=mon@entry=0x559905a7f460, str=0x7f5aa1eda010 "{\"return\": \"ffff9eb480000000-ffff9eb480099000 ", '0' <repeats 11 times>, "99000 -rw\\r\\nffff9eb480099000-ffff9eb48009b000 ", '0' <repeats 12 times>, "2000 -r-\\r\\nffff9eb48009b000-ffff9eb486800000 0000000006765000 -rw\\r\\nffff9eb4868000"...) at ./monitor/monitor.c:167
        i = 3865830
        c = <optimized out>
#6  0x0000559902f74342 in qmp_send_response (mon=0x559905a7f460, rsp=<optimized out>) at ./monitor/qmp.c:119
        data = <optimized out>
        json = 0x559906c88380
#7  0x0000559902f74409 in monitor_qmp_respond (rsp=0x559905bbf740, mon=0x559905a7f460) at ./monitor/qmp.c:132
        old_mon = <optimized out>
        rsp = 0x559905bbf740
        error = <optimized out>
#8  0x0000559902f74409 in monitor_qmp_dispatch (mon=0x559905a7f460, req=<optimized out>) at ./monitor/qmp.c:161
        old_mon = <optimized out>
        rsp = 0x559905bbf740
        error = <optimized out>
#9  0x0000559902f74bc0 in monitor_qmp_bh_dispatcher (data=<optimized out>) at ./monitor/qmp.c:234
        id = <optimized out>
        rsp = <optimized out>
        need_resume = true
        mon = 0x559905a7f460
        __PRETTY_FUNCTION__ = "monitor_qmp_bh_dispatcher"
#10 0x00005599030c37be in aio_bh_call (bh=0x559905571b40) at ./util/async.c:89
        bh = 0x559905571b40
        bhp = <optimized out>
        next = 0x5599055718f0
        ret = 1
        deleted = false
#11 0x00005599030c37be in aio_bh_poll (ctx=ctx@entry=0x5599055706f0) at ./util/async.c:117
        bh = 0x559905571b40
        bhp = <optimized out>
        next = 0x5599055718f0
        ret = 1
        deleted = false
#12 0x00005599030c6dd0 in aio_dispatch (ctx=0x5599055706f0) at ./util/aio-posix.c:459
#13 0x00005599030c369e in aio_ctx_dispatch (source=<optimized out>, callback=<optimized out>, user_data=<optimized out>) at ./util/async.c:260
        ctx = <optimized out>
#14 0x00007f5b6d37f417 in g_main_context_dispatch () at /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#15 0x00005599030c5e0a in glib_pollfds_poll () at ./util/main-loop.c:219
        context = 0x559905652420
        pfds = <optimized out>
        context = 0x559905652420
        ret = 1
        mlpoll = {state = 0, timeout = 4294967295, pollfds = 0x559905651800}
#16 0x00005599030c5e0a in os_host_main_loop_wait (timeout=14451267) at ./util/main-loop.c:242
        context = 0x559905652420
        ret = 1
        mlpoll = {state = 0, timeout = 4294967295, pollfds = 0x559905651800}
#17 0x00005599030c5e0a in main_loop_wait (nonblocking=<optimized out>) at ./util/main-loop.c:518
        mlpoll = {state = 0, timeout = 4294967295, pollfds = 0x559905651800}
#18 0x0000559902dd75df in main_loop () at ./vl.c:1810
#19 0x0000559902c59f49 in main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at ./vl.c:4466
        i = <optimized out>
        snapshot = 0
        linux_boot = <optimized out>
        initrd_filename = 0x0
        kernel_filename = <optimized out>
        kernel_cmdline = <optimized out>
        boot_order = 0x55990318bc94 "cad"
        boot_once = <optimized out>
        ds = <optimized out>
        opts = <optimized out>
        icount_opts = <optimized out>
        accel_opts = 0x0
        olist = <optimized out>
        optind = 100
        optarg = 0x7ffe0ca05e74 "timestamp=on"
        loadvm = 0x0
        cpu_option = 0x7ffe0ca05d42 "SandyBridge-IBRS,-kvm_steal_time,+pcid,+ssbd,+spec-ctrl,+md-clear"
        vga_model = 0x0
        qtest_chrdev = 0x0
        qtest_log = 0x0
        incoming = 0x0
        userconfig = <optimized out>
        nographic = false
        display_remote = <optimized out>
        log_mask = <optimized out>
        log_file = 0x0
        trace_file = <optimized out>
        maxram_size = <optimized out>
        ram_slots = 0
        vmstate_dump_file = 0x0
        main_loop_err = 0x0
        err = 0x0
        list_data_dirs = false
        dirs = <optimized out>
        bdo_queue = {sqh_first = 0x0, sqh_last = 0x7ffe0ca03540}
        plugin_list = {tqh_first = 0x0, tqh_circ = {tql_next = 0x0, tql_prev = 0x7ffe0ca03550}}
        __func__ = "main"