server: turn spice_server_migrate_start into a valid call
server: turn spice_server_migrate_start into a valid call

We will add a qemu call to spice_server_migrate_start when migration starts.
For now, it does nothing, but we may need this notification in the future.
(cherry picked from commit b8213167717979e6f2fb52646e43eb458634e6a1 branch 0.8)

freedesktop-unofficial-mirror/spice__spice,fgouget/spice,Fantu/spice,ya-mouse/spice,mathslinux/spice,Fantu/spice,elmarco/spice,elmarco/spice,fgouget/spice,dezelin/spice,fgouget/spice,SPICE/spice,Fantu/spice,mathslinux/spice,alon/spice-server,fgouget/spice,alon/spice-server,mathslinux/spice,mathslinux/spice,ya-mouse/spice,dezelin/spice,mathslinux/spice,elmarco/spice,dezelin/spice,elmarco/spice,SPICE/spice,alon/spice-server,Fantu/spice,alon/spice-server,ya-mouse/spice,freedesktop-unofficial-mirror/spice__spice,freedesktop-unofficial-mirror/spice__spice,freedesktop-unofficial-mirror/spice__spice,alon/spice-server,dezelin/spice,SPICE/spice,freedesktop-unofficial-mirror/spice__spice,elmarco/spice,dezelin/spice