This uses the new QEMU_CAPS_HOST_PCI_MULTIDOMAIN capability when
present, for -devivce pci-assign, -device vfio-pci, and -pcidevice.
While creating tests for this new functionality, I noticed that the
xmls for two existing tests had erroneously specified an
until-now-ignored domain="0x0002", so I corrected those two tests, and
also added two failure tests to be sure that we alert users who
attempt to use a non-zero domain with a qemu that doesn't support it.
