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 <[email protected]>
Message-Id: <20190718083747[email protected]>
Reviewed-by: Dr. David Alan Gilbert <[email protected]>
Signed-off-by: Dr. David Alan Gilbert <[email protected]>