Implement basic NIC hotplug support using the 'host_net_add' and
'pci_add' qemu monitor commands.

For now, we don't support 'bridge' or 'network' types.

Also, if pci_add fails, we currently fail to remove the backend
which we added.

Finally, NIC hot-unplug support is missing.

* src/qemu_driver.c: add qemudDomainAttachNetDevice()

* src/qemu_conf.[ch]: export qemuBuildNicStr(), qemuBuildHostNetStr()
  and qemuAssignNames()

* src/libvirt_private.syms: export virDomainNetTypeToString()