hyperv/vmbus: Flatten channel message response processing.

Sponsored by:   Microsoft OSTC
Differential Revision:  https://reviews.freebsd.org/D6914

hyperv/vmbus: Avoid tx_evtflags setting code duplication.

Sponsored by:   Microsoft OSTC
Differential Revision:  https://reviews.freebsd.org/D6915

hyperv/vmbus: Busdma-fy Hypercall signal event input parameter.

Sponsored by:   Microsoft OSTC
Differential Revision:  https://reviews.freebsd.org/D6916

hyperv: Nuke unused stuffs

Sponsored by:   Microsoft OSTC
Differential Revision:  https://reviews.freebsd.org/D6917

hyperv/vmbus: Don't be oversmart in default cpu selection.

Pin the channel to cpu0 by default.  Drivers having special channel-cpu
mapping requirement should call vmbus_channel_cpu_{set,rr}() themselves.

Sponsored by:   Microsoft OSTC
Differential Revision:  https://reviews.freebsd.org/D6918

hyperv/vmbus: Minor renaming

Sponsored by:   Microsoft OSTC
Differential Revision:  https://reviews.freebsd.org/D6919

hyperv/vmbus: Rework vmbus version accessing.

Instead of global variable, vmbus version is accessed through
a vmbus DEVMETHOD now.

Sponsored by:   Microsoft OSTC
Differential Revision:  https://reviews.freebsd.org/D6953

hyperv/vmbus: Move GPADL index into vmbus_softc

Sponsored by:   Microsoft OSTC
Differential Revision:  https://reviews.freebsd.org/D6954

hyperv/vmbus: Move channel list to vmbus_softc

Sponsored by:   Microsoft OSTC
Differential Revision:  https://reviews.freebsd.org/D6956