hyperv/vmbus: Use post message Hypercall APIs for channel request

Sponsored by:   Microsoft OSTC
Differential Revision:  https://reviews.freebsd.org/D6831

hyperv/vmbus: Function renaming.

And pass vmbus_softc to vmbus_doattach()

Sponsored by:   Microsoft OSTC
Differential Revision:  https://reviews.freebsd.org/D6842

hyperv/vmbus: Explicitly assign channel message process array.

While I'm here, remove the useless message type from message process
array, which is not used and serves no purposes at all.

Sponsored by:   Microsoft OSTC
Differential Revision:  https://reviews.freebsd.org/D6858

hyperv/vmbus: Add sysctl to expose vmbus version.

Requested by:   Hongxiong Xian <v-hoxian microsoft com>
Sponsored by:   Microsoft OSTC
Differential Revision:  https://reviews.freebsd.org/D6860

hyperv/vmbus: Use post message Hypercall APIs for unload

Sponsored by:   Microsoft OSTC
Differential Revision:  https://reviews.freebsd.org/D6861

hyperv/vmbus: Create channel synchronously.

The device probe/attach has been move to a different thread, so the
reasons to create the channel asynchronously are no longer valid.

Sponsored by:   Microsoft OSTC
Differential Revision:  https://reviews.freebsd.org/D6862

hyperv/vmbus: Save vmbus softc to channels.

So that we don't need to access the global vmbus softc.

Sponsored by:   Microsoft OSTC
Differential Revision:  https://reviews.freebsd.org/D6863

hyperv/vmbus: Embed channel detach task in channel itself.

GC work queue stuffs.

Sponsored by:   Microsoft OSTC
Differential Revision:  https://reviews.freebsd.org/D6864

hyperv/vmbus: Reorganize vmbus scan process.

Sponsored by:   Microsoft OSTC
Differential Revision:  https://reviews.freebsd.org/D6875