hyperv/vmbus: add a new method to get vcpu_id

vcpu_id is host's representation of guest CPU.
We get the mapping between vcpu_id and FreeBSD kernel's cpu id when VMBus
driver is loaded. Later, when a driver, like the coming pcib driver, talks
to the host and needs to refer to a guest CPU, the driver must use the
vcpu_id.

Reviewed by: jhb, sephe
Approved by: sephe (mentor)
MFC after: 1 week
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D8410