spice: adding seamless-migration option to the command line

spice: adding seamless-migration option to the command line

The seamless-migration flag is required in order to identify
whether libvirt supports the new QEVENT_SPICE_MIGRATE_COMPLETED or not
(by default the flag is off).
New libvirt versions that wait for QEVENT_SPICE_MIGRATE_COMPLETED should turn on this flag.
When this flag is off, spice fallbacks to its old migration method, which
can result in data loss.

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