tests/qtest: enhance migration channels

tests/qtest: enhance migration channels

Change the migrate_qmp and migrate_qmp_fail channels argument to a QObject
type so the caller can manipulate the object before passing it to the
helper.  Define migrate_str_to_channel to aid such manipulation.
Add a channels argument to migrate_incoming_qmp.

Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/1736967650-129648-22-git-send-email-steven.sistare@oracle.com
Signed-off-by: Fabiano Rosas <farosas@suse.de>