migration/postcopy: start_postcopy could be true only when migrate_postcopy() return...

migration/postcopy: start_postcopy could be true only when migrate_postcopy() return true

There is only one place to set start_postcopy to true,
qmp_migrate_start_postcopy(), which make sure start_postcopy could be
set to true when migrate_postcopy() return true.

So start_postcopy is true implies the other one.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Message-Id: <20190718083747.5859-1-richardw.yang@linux.intel.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>