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