tests/qtest/migration: consolidate set capabilities

tests/qtest/migration: consolidate set capabilities

Migration capabilities are set in multiple '.start_hook'
functions for various tests. Instead, consolidate setting
capabilities in 'migrate_start_set_capabilities()' function
which is called from the 'migrate_start()' function.
While simplifying the capabilities setting, it helps
to declutter the qtest sources.

Suggested-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Prasad Pandit <pjp@fedoraproject.org>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Message-ID: <20250411114534.3370816-7-ppandit@redhat.com>
[fix open brace]
Signed-off-by: Fabiano Rosas <farosas@suse.de>