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()