Deadlock on snapshot removal (bdrv_graph_wrlock)
Description of problem:
VM was hanging during snapshot removal.
There was an attempt to shutdown the VM, but that did hang.

gdb shows me:
```
(gdb) bt full
#0  0x00007f20493427fe in __ppoll (fds=0x557e630718b0, nfds=2, timeout=<optimized out>, sigmask=0x0) at ../sysdeps/unix/sysv/linux/ppoll.c:43
        sc_ret = -514
        sc_cancel_oldtype = 0
        sc_ret = <optimized out>
        tval = {tv_sec = 139776632323420, tv_nsec = 139776632323432}
#1  0x0000557e619cab52 in fdmon_poll_wait.llvm ()
No symbol table info available.
#2  0x0000557e619ca0b6 in aio_poll ()
No symbol table info available.
#3  0x0000557e61801651 in bdrv_graph_wrlock ()
No symbol table info available.
#4  0x0000557e617c873b in bdrv_replace_child_noperm.llvm ()
No symbol table info available.
#5  0x0000557e617c8601 in bdrv_root_unref_child ()
No symbol table info available.
#6  0x0000557e617f6333 in blk_unref ()
No symbol table info available.
#7  0x0000557e6181b0d1 in mirror_exit_common ()
No symbol table info available.
#8  0x0000557e617dbdb4 in job_do_finalize_locked.llvm ()
No symbol table info available.
#9  0x0000557e617dd72b in job_exit ()
No symbol table info available.
#10 0x0000557e619e5101 in aio_bh_poll ()
No symbol table info available.
#11 0x0000557e619c95a4 in aio_dispatch ()
No symbol table info available.
#12 0x0000557e619e655f in aio_ctx_dispatch ()
No symbol table info available.
#13 0x00007f2049546e2f in g_main_dispatch (context=0x557e62ecebd0) at ../glib/gmain.c:3337
        dispatch = 0x557e619e6550 <aio_ctx_dispatch>
        prev_source = 0x0
        begin_time_nsec = 232172181173336
        was_in_call = <optimized out>
        user_data = 0x0
        callback = 0x0
        cb_funcs = 0x0
        cb_data = 0x0
        need_destroy = <optimized out>
        source = 0x557e62ec73e0
        current = 0x557e63e4b600
        i = 0
        __func__ = {<optimized out> <repeats 16 times>}
#14 g_main_context_dispatch (context=0x557e62ecebd0) at ../glib/gmain.c:4055
No locals.
#15 0x0000557e619e74be in main_loop_wait ()
No symbol table info available.
#16 0x0000557e615201e7 in qemu_main_loop ()
No symbol table info available.
#17 0x0000557e61374c6a in qemu_default_main ()
No symbol table info available.
#18 0x00007f204923feb0 in __libc_start_call_main (main=main@entry=0x557e61374c80 <main>, argc=argc@entry=153, argv=argv@entry=0x7ffe07495238) at ../sysdeps/nptl/libc_start_call_main.h:58
        self = <optimized out>
        result = <optimized out>
        unwind_buf = {cancel_jmp_buf = {{jmp_buf = {0, -6145724763479124305, 140729020666424, 94001285254272, 94001294953808, 139776661151744, 6145708635144279727, 6121919821307926191}, mask_was_saved = 0}}, priv = {pad = {0x0, 0x0, 0x7f204954cb41 <g_malloc0+33>, 0x0}, data = {prev = 0x0, cleanup = 0x0, canceltype = 1230293825}}}
        not_first_call = <optimized out>
#19 0x00007f204923ff60 in __libc_start_main_impl (main=0x557e61374c80 <main>, argc=153, argv=0x7ffe07495238, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7ffe07495228) at ../csu/libc-start.c:389
No locals.
#20 0x0000557e613743d5 in _start ()
No symbol table info available.
```
Steps to reproduce:
Still trying to reproduce in lab.