fix broken migration

fix broken migration

While fixing migration with -S, commit
89befdd1a6b18215153b8976682d57b7d03d5782 broke the rest of us. Poor
glommer, with a poor family, spare him his life from this monstruosity.

Since the unconditional vm_start, not autostart was the villain, I'm putting
back autostart. Let me know if you prefer other solutions, it doesn't really matter,
doesn't really matter to me.

Any way the wind blows...

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