Add missing sanity check.
Reproducer: run qemu with "-device qxl" but without "-spice ..."

Fixes: 0d9b90ce ("console: make QMP/HMP screendump run in coroutine")
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20201124122936.30588-1-kraxel@redhat.com