hyperv: vmbus: remove the per-channel control_work_queue

Now vmbus_channel_on_offer() -> vmbus_channel_process_offer() can
safely run on the global hv_vmbus_g_connection.work_queue now.

We remove the per-channel control_work_queue to achieve the proper
serialization of the message handling.

I removed the bogus TODO in vmbus_channel_on_offer(): a vmbus offer
can only come from the parent partition, i.e., the host.

PR: kern/205156
Submitted by: Dexuan Cui <decui microsoft com>
Reviewed by: Howard Su <howard0su gmail com>, delphij
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D4597