hyperv/hn: Fix primary channel revocation

Since hypervisor will not drain the TX bufring, once the channels are
revoked:
- Setup vmbus orphan handler properly.
- Make sure that suspension will not wait the TX bufring draining
  forever.
- GC the pending TX descs on detach path, before freeing the busdma
  stuffs.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D8559