The QEMU 0.12.x tree has the -netdev command line argument, but not
corresponding monitor command. We can't enable the former, without
the latter since it will break hotplug/unplug.

* src/qemu/qemu_conf.c, src/qemu/qemu_conf.h: Disable -netdev usage
  until 0.13 at earliest
* tests/qemuxml2argvtest.c: Add test for -netdev syntax
* tests/qemuxml2argvdata/qemuxml2argv-net-virtio-netdev.args,
  tests/qemuxml2argvdata/qemuxml2argv-net-virtio-netdev.xml: Test
  data files for -netdev syntax