tests/qtest/migration: Add channels parameter in migrate_qmp

tests/qtest/migration: Add channels parameter in migrate_qmp

Alter migrate_qmp() to allow use of channels parameter, but only
fill the uri with correct port number if there are no channels.
Here we don't want to allow the wrong cases of having both or
none (ex: migrate_qmp_fail).

Signed-off-by: Het Gala <het.gala@nutanix.com>
Suggested-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240312202634.63349-7-het.gala@nutanix.com
Signed-off-by: Peter Xu <peterx@redhat.com>