migration: Centralize BH creation and dispatch

migration: Centralize BH creation and dispatch

Now that the migration state reference counting is correct, further
wrap the bottom half dispatch process to avoid future issues.

Move BH creation and scheduling together and wrap the dispatch with an
intermediary function that will ensure we always keep the ref/unref
balanced.

Also move the responsibility of deleting the BH into the wrapper and
remove the now unnecessary pointers.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240119233922.32588-6-farosas@suse.de
Signed-off-by: Peter Xu <peterx@redhat.com>