tests: Pass in MigrateStart** into test_migrate_start()

tests: Pass in MigrateStart** into test_migrate_start()

test_migrate_start() will release the MigrateStart structure that passed
in, however that's not super clear to the caller because after the call
returned the pointer can still be referenced by the callers.  It can easily
be a source of use-after-free.

Let's pass in a double pointer of that, then we can safely clear the
pointer for the caller after the struct is released.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20220301083925.33483-26-peterx@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
  dgilbert: Fixup apply since I didn't take 24/25