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

[Jim Paris <jim jtan com>]

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