qemu: use vfio-pci on commandline when appropriate

qemu: use vfio-pci on commandline when appropriate

The device option for vfio-pci is nearly identical to that for
pci-assign - only the configfd parameter isn't supported (or needed).

Checking for presence of the bootindex parameter is done separately
from constructing the commandline, similar to how it is done for
pci-assign.

This patch contains tests to check for proper commandline
construction. It also includes tests for parser-formatter-parser
roundtrips (xml2xml), because those tests use the same data files, and
would have failed had they been included before now.

qemu: xml/args tests for VFIO hostdev and <interface type='hostdev'/>

These should be squashed in with the patch that adds commandline
handling of vfio (they would fail at any earlier time).