tests/qtest/migration: Move ufd_version_check to utils

tests/qtest/migration: Move ufd_version_check to utils

Move ufd_version_check() to migration-util.c file. This is a helper
function that is used during tests definition so it should be
available outside of migration-test.c

Since the function is moving to another file, change it to stop
accessing the global uffd_feature_thread_id and take it as argument
instead.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>