migration: Refactor auto-converge capability logic

migration: Refactor auto-converge capability logic

Check if block migration is running before throttling
guest down in auto-converge way.

Note that this modification is kind of like code clean,
because block migration does not depend on auto-converge
capability, so the order of checks can be adjusted.

Signed-off-by: Hyman Huang(黄勇) <yong.huang@smartx.com>
Acked-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <168618975839.6361.17407633874747688653-5@git.sr.ht>
Signed-off-by: Juan Quintela <quintela@redhat.com>