migration: Put thread names together with macros

migration: Put thread names together with macros

Keep migration thread names together, so it's easier to see a list of all
possible migration threads.

Still two functional changes below besides the macro defintions:

  - There's one dirty rate thread that we overlooked before, now we add
  that too and name it as "mig/dirtyrate" following the old rules.

  - The old name "mig/src/rp-thr" has "-thr" but it may not be useful if
  it's a thread name anyway, while "rp" can be slightly hard to read.
  Taking this chance to rename it to "mig/src/return", hopefully a better
  name.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Acked-by: Hyman Huang <yong.huang@smartx.com>
Reviewed-by: Zhang Chen <chen.zhang@intel.com>
Link: https://lore.kernel.org/r/20241011153652.517440-1-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>