spice: turn client_migrate_info to async

spice: turn client_migrate_info to async

RHBZ 737921
Spice client is required to connect to the migration target before/as migration
starts. Since after migration starts, the target qemu is blocked and cannot accept new spice client
we trigger the connection to the target upon client_migrate_info command.
client_migrate_info completion cb will be called after spice client has been
connected to the target (or a timeout). See following patches and spice patches.

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