Avoid permanently disabled QEMU monitor when UNIX migration fails

If a UNIX migration command is attempt to a UNIX socket which does
not exist, then the monitor is suspended, but never resumed. This
prevents any further use of the monitor

* migration-unix.c: Only call migrate_fd_monitor_suspend() once
  connected to the UNIX socket.

   Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
