hyperv/vmbus: add a new method to get vcpu_id

[decui_microsoft.com]

Summary

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.

[sepherosa_gmail.com]

sys/dev/hyperv/vmbus/vmbus.c

999–1001

Or:
const struct vmbus_softc *sc = device_get_softc(bus);

return (VMBUS_PCPU_GET(sc, vcpuid, cpu));

[decui_microsoft.com]

This new version addressed Sephe's comment.

[sepherosa_gmail.com]

Ship it!