spice: wire up monitor in QemuSpiceOps.

Rename qmp_query_spice() to qmp_query_spice_real(), add to QemuSpiceOps.

Add new qmp_query_spice() function which calls the real function via
QemuSpiceOps if available, otherwise return SpiceInfo.enabled = false.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20201019075224.14803-9-kraxel@redhat.com