hyperv/vmbus: Simplify event processing

For channel0, it will never be processed on event handling path,
so there is no need to install it. After skipping in the channel0
installation, we could discard the channel0 check on event
handling hot code path.

MFC after: 1 week
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D6333