hyperv/vmbus: Fix the multi-channel revoking on vmbus side.

hyperv/vmbus: Fix the multi-channel revoking on vmbus side.

- Reference count the sub-channel when channel offer message is
  processed, so that immediate rescind message on the same channel
  will not race sub-channel open on driver side.
- Drop the above reference when sub-channel is closed, this closely
  mimics the hypervisor's reaction when primary channel is closed
  on the VM side.  No drivers use sub-channel after primary channel
  is closed.

MFC after:	1 week
Sponsored by:	Microsoft
Differential Revision:	https://reviews.freebsd.org/D8546