VNC UNIX sockets are not deleted
Description of problem:
After exiting QEMU a unix VNC socket file is left behind. Upon termination I would expect it to remove the socket file like it does for example with a monitor unix socket.
Steps to reproduce:
```
   rm -f foo.socket
   qemu-system-x86_64 -vnc unix:foo.socket
   # Exit QEMU
   ls foo.socket
   ```