VNC unix-domain socket unlink()ed prematurely

With qemu 3.0.0 (I don't believe this happened with previous
versions), if I tell it `-vnc unix:/path/to/vnc.sock`, qemu will
unlink() that file when the first client disconnects, meaning that
once I disconnect, I can't ever reconnect without restarting the VM.

A stupid testcase demonstrating the issue:

In terminal A:

    $ qemu-system-x86_64 -vnc unix:$PWD/vnc.sock

In terminal B:

    $ ls vnc.sock
    vnc.sock
    $ socat STDIO UNIX-CONNECT:vnc.sock <<<''
    RFB 003.008
    $ ls vnc.sock
    ls: cannot access 'vnc.sock': No such file or directory

I have determined that the offending unlink() call is the one in
io/channel-socket.c:qio_channel_socket_close().  That call was first
introduced in commit d66f78e1eaa832f73c771d9df1b606fe75d52a50, which
first appeared in version 3.0.0.

This type of premature unlink() does not happen on monitor.sock with
`-monitor unix:/path/to/monitor.sock,server,nowait`.

I am not familiar enough with the QIO subsystem to suggest a fix that
fixes VNC, but preserves the QMP fix targeted in the offending commit.

This is still a problem with 3.1.0.

Added Daniel to the bug.

It only affects VNC, not chardevs because the chardevs fail to call qio_channel_close() and just rely on finalize() cleaning up the open socket. To fix this we just need to made the code conditional on it being a listener socket

  if (qio_channel_has_feature(ioc, QIO_CHANNEL_FEATURE_LISTEN)) {
   ...
  }

Patch proposed at

https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg02774.html

Fix merged to git master  https://git.qemu.org/?p=qemu.git;a=commit;h=feff02089113839d233e40a9bd7134241de12d1d