hyperv/vmbus: Fix the primary channel revoking on vmbus side.

Drivers can now use vmbus_chan_{is_revoked,set_orphan,unset_orphan}() and
vmbus_xact_ctx_orphan() to fix their attach/detach DEVMETHODs for revoked
primary channels.

MFC after: 1 week
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D8545