dump-guest-memory: Block live migration

dump-guest-memory: Block live migration

Both dump-guest-memory and live migration caches vm state at the beginning.
Either of them entering the other one will cause race on the vm state, and even
more severe on that (please refer to the crash report in the bug link).

Let's block live migration in dump-guest-memory, and that'll also block
dump-guest-memory if it detected that we're during a live migration.

Side note: migrate_del_blocker() can be called even if the blocker is not
inserted yet, so it's safe to unconditionally delete that blocker in
dump_cleanup (g_slist_remove allows no-entry-found case).

Suggested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1996609
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>