hyperv/vmbus: Add result polling support for xact API.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D8633

hyperv/vmbus: Add result polling support for message Hypercall API.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D8634

hyperv/vmbus: Add exec cancel support for message Hypercall API.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D8635

hyperv/vmbus: Use poll/cancel APIs to wait for the CHOPEN response.

Since hypervisor does not respond CHOPEN to a revoked channel.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D8636

hypver/vmbus: Remove extra assertion.

It is asserted by vmbus_chan_gpadl_connect() now.

Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D8660

hyperv/vmbus: Use pause if possible.

This makes booting on Hyper-V w/ small # of vCPUs work properly.

Reported by:    Hongxiong Xian <v-hoxian microsoft com>, Hongjiang Zhang <honzhan microsoft com>
Sponsored by:   Microsoft