hyperv/vmbus: Cosmetic bufring cleanup.

Sponsored by:   Microsoft OSTC
Differential Revision:  https://reviews.freebsd.org/D7267

hyperv/vmbus: Cleanup and augment bufring sysctl tree creation

Binary state node is added, so that userland programs do not have
to parse human readable state string.

Sponsored by:   Microsoft OSTC
Differential Revision:  https://reviews.freebsd.org/D7268

hyperv/vmbus: Move vmbus bufring definition to vmbus_reg.h

And add more comment about its fields.

Sponsored by:   Microsoft OSTC
Differential Revision:  https://reviews.freebsd.org/D7269