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
