tests/qtest/migration: Add channels parameter in migrate_qmp_fail

tests/qtest/migration: Add channels parameter in migrate_qmp_fail

Alter migrate_qmp_fail() to allow both uri and channels
independently. For channels, convert string to a Dict.
No dealing with migrate_get_socket_address() here because
we will fail before starting the migration anyway.

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/20240312202634.63349-5-het.gala@nutanix.com
Signed-off-by: Peter Xu <peterx@redhat.com>