Into qemu_using_spice().  For want of a better place, put it next the
existing monitor command handler dummies in qemu-spice.h.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>