Support virtio-serial controller and virtio channel in QEMU backend.
Will output
the following for virtio-serial controller:

-device
virtio-serial-pci,id=virtio-serial0,bus=pci.0,addr=0x4,max_ports=16,vectors=4

and the following for a virtio channel:

-chardev pty,id=channel0 \
-device
virtserialport,bus=virtio-serial0.0,chardev=channel0,name=org.linux-kvm.port.0

* src/qemu/qemu_conf.c: Add argument output for virtio
* tests/qemuxml2argvtest.c
  tests/qemuxml2argvdata/qemuxml2argv-channel-virtio.args: Add test for
  QEMU command line generation