if_vmove: improve restoration in cloner's ifgroup membership

• Do a single call into if_clone.c instead of two. The cloner can't disappear since the interface sits on its list.
• Make restoration smarter - check that cloner with same name exists in the new vnet.

Differential revision: https://reviews.freebsd.org/D33941

(cherry picked from commit 54712fc42350acb4991d8bd8604b08562660e962)