migration: cleanup check on ops in savevm.handlers iterations

During migration, there are several places to iterate on
savevm.handlers. And on each iteration, we need to check its ops and
related callbacks before invoke it.

Generally, ops is the first element to check, and it is only necessary
to check it once.

This patch clean all the related part in savevm.c to check ops only once
in those iterations.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>

Message-Id: <20190819032804.8579-1-richardw.yang@linux.intel.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>