migration: Cleanup migrate_fd_cleanup() on accessing to_dst_file

migration: Cleanup migrate_fd_cleanup() on accessing to_dst_file

The cleanup function can in many cases needs cleanup on its own.

The major thing we want to do here is not referencing to_dst_file when
without the file mutex.  When at it, touch things elsewhere too to make it
look slightly better in general.

One thing to mention is, migration_thread has its own "running" boolean, so
it doesn't need to rely on to_dst_file being non-NULL.  Multifd has a
dependency so it needs to be skipped if to_dst_file is not yet set; add a
richer comment for such reason.

Resolves: Coverity CID 1527402
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240919163042.116767-1-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>