honor -S on incoming migration

-S is not honored by qemu on incoming migration.  If a domain is migrated
while paused, thus, it will start running on the remote machine; this
is wrong.

Given the trivial patch to fix this, it looks more like a thinko
than anything else, probably dating back to the qemu-kvm merge.
The interesting part is that the -S mechanism was in fact *used* when
migrating (setting autostart = 0) and the incoming migration code was
starting the VM at the end of the migration.

Since I was removing the vm_start from there, I also corrected a related
imprecision.  The code was doing a vm_stop "just in case", but we can
be sure that the VM is not running---the vm_start call in vl.c has not
been reached yet.  So the vm_stop is removed together with the vm_start.

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