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)
Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D8410

hyperv/vmbus: add new vmbus methods to support PCIe pass-through

The new methods will be used by the coming pcib driver.

Reviewed by:    sephe
Approved by:    sephe (mentor)
Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D8409

hyperv/pcib: enable PCIe pass-through (a.k.a. Discrete Device Assignment)

The feature enables us to pass through physical PCIe devices to FreeBSD VM
running on Hyper-V (Windows Server 2016) to get near-native performance with
low CPU utilization.

The patch implements a PCI bridge driver to support the feature:

1) The pcib driver talks to the host to discover device(s) and presents
the device(s) to FreeBSD's pci driver via PCI configuration space (note:
to access the configuration space, we don't use the standard I/O port
0xCF8/CFC method; instead, we use an MMIO-based method supplied by Hyper-V,
which is very similar to the 0xCF8/CFC method).

2) The pcib driver allocates resources for the device(s) and initialize
the related BARs, when the device driver's attach method is invoked;

3) The pcib driver talks to the host to create MSI/MSI-X interrupt
remapping between the guest and the host;

4) The pcib driver supports device hot add/remove.

Reviewed by:    sephe
Approved by:    sephe (mentor)
Sponsored by:   Microsoft
Differential Revision:  https://reviews.freebsd.org/D8332

hyperv/pcib: Fix the build for some kernel configs

Add the dependency on pci explicitly for the pcib and vmbus drivers.
The related Makefiles are updated accordingly too.

Reviewed by:    sephe
Approved by:    sephe (mentor)
Sponsored by:   Microsoft

hyperv/vmbus,pcib: Add MODULE_DEPEND on pci

We'd better add this dependency explicitly, though usually the pci
driver is built into the kernel by default.

Reviewed by:    sephe
Approved by:    sephe (mentor)
Sponsored by:   Microsoft

hyperv/pcib: change the file name: pcib.c -> vmbus_pcib.c

This makes the file name and the variable naming in the file consistent.

Reviewed by:    sephe
Approved by:    sephe (mentor)
Sponsored by:   Microsoft

hyperv/vmbus,pcib: unbreak build in case NEW_PCIB is undefined

vmbus_pcib requires NEW_PCIB, but in case that's not defined, we at
least shouldn't break build.

Reviewed by:    sephe
Approved by:    sephe (mentor)
Sponsored by:   Microsoft