qxl: upon reset, if spice worker is stopped, the command rings can be not empty

Spice worker does no longer process commands when it is stopped.
Otherwise, it might crash during migration when attempting to process
commands while the guest is not completely loaded.

Cc: Alon Levy <alevy@redhat.com>

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
