[Libvir] PATCH: Support NIC model selection for QEMU/KVM

Hi, Sometime between kvm-36 and kvm-46 I ran into problems with the default QEMU network card (ne2k-pci). Switching it fixed the problems, but libvirt doesn't support changing the NIC model. These patches add support for: <interface> <nic model="rtl8139"/> </interface> which becomes qemu -net nic,model=rtl8139,mac=... By default, no model is appended to the qemu command line, as before. Documentation update & some fixes are included too. -jim