libxl: only query VNC when enabled

libxl: only query VNC when enabled

QEMU without VNC support (configure --disable-vnc) will return an error
when VNC is queried over QMP since it does not recognize the QMP
command.  This will cause libxl to fail starting the domain even if VNC
is not enabled.  Therefore only query QEMU for VNC support when using
VNC, so a VNC-less QEMU will function in this configuration.

'goto out' jumps to the call to device_model_postconfig_done(), the
final callback after the chain of vnc queries.  This bypasses all the
QMP VNC queries.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Acked-by: Wei Liu <wl@xen.org>