migration: refactor channel discovery mechanism

migration: refactor channel discovery mechanism

The various logical migration channels don't have a
standardized way of advertising themselves and their
connections may be seen out of order by the migration
destination. When a new connection arrives, the incoming
migration currently make use of heuristics to determine
which channel it belongs to.

The next few patches will need to change how the multifd
and postcopy capabilities interact and that affects the
channel discovery heuristic.

Refactor the channel discovery heuristic to make it less
opaque and simplify the subsequent patches.

Signed-off-by: Prasad Pandit <pjp@fedoraproject.org>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Message-ID: <20250411114534.3370816-3-ppandit@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>