[libvirt] [PATCH 0/6] qemu: add suport for "romfile", and associated cleanups

This patchset started out as cleanup to make it easier to add in support for doing PCI passthrough via the <interface> element (to be used to attach a host network device to a guest after performing some network-device-specific setup), but turned out to also make it much easier to add support for QEMU's "romfile" option, as requested in https://bugzilla.redhat.com/show_bug.cgi?id=781562 Patch 1 is trivial, and probably qualifies for pushing under the trivial rule, but it's just as easy to leave it in with the rest. Patches 2-4 reorganize some data definitions, and the code associated with them, with no functional change. Patch 5 expands support for "rombar" to network devices (as long as they attach to the guest PCI bus), and Patch 6 adds support for "romfile" to both generic PCI devices and network devices.