qemu: fix UNIX socket chardevs operating in client mode

qemu: fix UNIX socket chardevs operating in client mode

When support was adding for passing a pre-opened listener socket to UNIX
chardevs, it accidentally passed the listener socket for client mode
chardevs too with predictable amounts of fail resulting. This affects
libvirt when using QEMU >= 2.12

Expand the unit test coverage to validate that we are only doing FD
passing when operating in server mode.

  https://bugzilla.redhat.com/show_bug.cgi?id=1598440

Tested-by: Richard W.M. Jones <rjones@redhat.com>
Reported-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>