fix migration to obey -S

Since migration returns right away, starting the VM right
after calling qemu_start_incoming_migration is wrong even
if -S is not passed.  We have to do this after migration
has completed.

Cc: Glauber Costa  <glommer@redhat.com>
Cc: Anthony Liguori  <aliguori@us.ibm.com>

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Glauber Costa <glommer@redhat.com>