hyperv/vmbus: Set vcpuid to 0, if MSR_HV_VP_INDEX does not exist.

Mainly for compatibility.  While I'm here, rename cpuid related
fields in hv_vmbus_channel.

Sponsored by:   Microsoft OSTC
Differential Revision:  https://reviews.freebsd.org/D7141

hyperv/vmbus: Field rename

Sponsored by:   Microsoft OSTC
Differential Revision:  https://reviews.freebsd.org/D7146