migration: Move migrate_use_xbzrle() to options.c

Once that we are there, we rename the function to migrate_xbzrle()
to be consistent with all other capabilities.
We change the type to return bool also for consistency.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>