tests/qtest/migration: Add multifd_tcp_plain test using list of channels instead...

tests/qtest/migration: Add multifd_tcp_plain test using list of channels instead of uri

Add a positive test to check multifd live migration but this time
using list of channels (restricted to 1) as the starting point
instead of simple uri string.

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/20240312202634.63349-8-het.gala@nutanix.com
Signed-off-by: Peter Xu <peterx@redhat.com>