There is really no difference between live migration and savevm, except
that savevm does not require bdrv_invalidate_cache to be implemented
by all disks.  However, it is unlikely that savevm is used with anything
except qcow2 disks, so the penalty is small and worth the improvement
in catching bad usage of savevm.

Only one place was taking care of savevm when adding a migration blocker,
and it can be removed.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>