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.

MFC after: 1 week
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D8559