Add a method to get the PCI RID for a device
Re-implement the DMAR I/O MMU code in terms of PCI RIDs

Under the hood the VT-d spec is really implemented in terms of
PCI RIDs instead of bus/slot/function, even though the spec makes
pains to convert back to bus/slot/function in examples.  However
working with bus/slot/function is not correct when PCI ARI is
in use, so convert to using RIDs in most cases.  bus/slot/function
will only be used when reporting errors to a user.
Re-write bhyve's I/O MMU handling in terms of PCI RIDs
Add support for PCIe ARI

PCIe Alternate RID Interpretation (ARI) is an optional feature that
allows devices to have up to 256 different functions.  It is
implemented by always setting the PCI slot number to 0 and
re-purposing the 5 bits used to encode the slot number to instead
contain the function number.  Combined with the original 3 bits
allocated for the function number, this allows for 256 functions.
Print status of ARI capability in pciconf -c