qxl: Call qxl_gem_{init, fini}

qdev->gem.objects was initialized directly in qxl_device_init() rather
than going through qxl_gem_init(), and qxl_gem_fini() was never called.

Signed-off-by: Christophe Fergeau <cfergeau@redhat.com>
Message-id: 20161108091209.25568-5-cfergeau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>