migration: Rename thread debug names

migration: Rename thread debug names

The postcopy thread names on dest QEMU are slightly confusing, partly I'll
need to blame myself on 36f62f11e4 ("migration: Postcopy preemption
preparation on channel creation").  E.g., "fault-fast" reads like a fast
version of "fault-default", but it's actually the fast version of
"postcopy/listen".

Taking this chance, rename all the migration threads with proper rules.
Considering we only have 15 chars usable, prefix all threads with "mig/",
meanwhile identify src/dst threads properly this time.  So now most thread
names will look like "mig/DIR/xxx", where DIR will be "src"/"dst", except
the bg-snapshot thread which doesn't have a direction.

For multifd threads, making them "mig/{src|dst}/{send|recv}_%d".

We used to have "live_migration" thread for a very long time, now it's
called "mig/src/main".  We may hope to have "mig/dst/main" soon but not
yet.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Zhijian Li (Fujitsu) <lizhijian@fujitsu.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>