Refactor PCI device creation

Refactor creation of PCI devices into helper methods that can be
used by the VF creation code.
Refactor PCI resource allocation

Refactor PCI resource allocation code to allow a request for a
memory-mapped I/O window that is a multiple of a requested size.
This is needed by the SR-IOV code because the VF BARs are all
allocated contiguously.  We can't just allocate a resource that is
a multiple of a single VF BAR because the size of an allocation
implies its alignment requirement.
Add some pcib methods to get ARI-related information
Implement interface to create SR-IOV Virtual Functions

Implement the interace to create SR-IOV Virtual Functions (VFs).
When a driver registers that they support SR-IOV by calling
pci_setup_iov(), the SR-IOV code creates a new node in /dev/iov
for that device.  An ioctl can be invoked on that device to
create VFs and have the driver initialize them.

At this point, allocating memory I/O windows (BARs) is not
supported.
Allow passthrough devices to be hinted.

Allow the ppt driver to attach to devices that were hinted to be
passthrough devices by the PCI code creating them with a driver
name of "ppt".

Add a tunable that allows the IOMMU to be forced to be used.  With
SR-IOV passthrough devices the VFs may be created after vmm.ko is
loaded.  The current code will not initialize the IOMMU in that
case, meaning that the passthrough devices can't actually be used.
Emulate the Device ID and Vendor ID registers for VFs

The SR-IOV standard requires VFs to read all-ones when the VID
and DID registers are read.  The VMM (hypervisor) is required to
emulate them instead.  Make pci_read_config() do this emulation.

Change pci_user.c to use pci_read_config() to read config space
registers instead of going directly to the pcib so that the
emulated VID/DID registers work correctly on VFs.  This is
required both for pciconf and bhyte PCI passthrough.
Allocate PCI I/O memory spaces for VFs

When creating VFs, we must size each SR-IOV BAR on the PF and
allocate a configuous I/O memory window large enough for every VF.
However, the window only needs to be aligned to a boundary equal
to the size of the window for a single VF.

When a VF attempts to allocate an I/O memory resource, we must
intercept the request in the pci driver and pass it off to the
SR-IOV code, which will allocate the correct window from the
pre-allocated memory space for the PF.

Inform the pci driver about the size and address of the BARs on
the VF when the VF is created.  This is required by pciconf -b and
bhyve.