Add support for virtio-serial by defining a new 'virtio' channel target type
and a virtio-serial controller. Allows the following to be specified in a
domain:
<controller type='virtio-serial' index='0' ports='16' vectors='4'/>
<channel type='pty'>
<target type='virtio' name='org.linux-kvm.port.0'/>
<address type='virtio-serial' controller='0' bus='0'/>
</channel>
* docs/schemas/domain.rng: Add virtio-serial controller and virtio
channel type.
* src/conf/domain_conf.[ch]: Domain parsing/serialization for
virtio-serial controller and virtio channel.
* tests/qemuxml2xmltest.c
tests/qemuxml2argvdata/qemuxml2argv-channel-virtio.xml: add domain xml
parsing test
* src/libvirt_private.syms src/qemu/qemu_conf.c:
virDomainDefAddDiskControllers() renamed to
virDomainDefAddImplicitControllers()
