migration: fix coverity migrate_mode finding

migration: fix coverity migrate_mode finding

Coverity diagnoses a possible out-of-range array index here ...

    static GSList *migration_blockers[MIG_MODE__MAX];

    fill_source_migration_info() {
        GSList *cur_blocker = migration_blockers[migrate_mode()];

... because it does not know that MIG_MODE__MAX will never be returned as
a migration mode.  To fix, assert so in migrate_mode().

Fixes: fa3673e497a1 ("migration: per-mode blockers")
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/1699907025-215450-1-git-send-email-steven.sistare@oracle.com
Signed-off-by: Peter Xu <peterx@redhat.com>