Fix test breakage from virtio serial changes

Fix test breakage from virtio serial changes

The virtio serial changes broke the test suite because they forgot
to add the new address attribute to the domain XML schema. The
xml2xml test also broke because the XML no longer roundtrips. This
is due to testing of auto-addition of <controller> elements. Split
that test case off into a separate XML file to avoid breakage

* docs/schemas/domain.rng: Allow port number for virtio serial addresses
* tests/qemuxml2argvdata/qemuxml2argv-channel-virtio.args,
  tests/qemuxml2argvdata/qemuxml2argv-channel-virtio.xml: Revert to
  a simple config to avoid breaking xml2xml test
* tests/qemuxml2argvdata/qemuxml2argv-channel-virtio-auto.xml,
  tests/qemuxml2argvdata/qemuxml2argv-channel-virtio-auto.args: Add
  complex test case for auto-controller addition for xml2argv test
* tests/qemuxml2argvtest.c: Add channel-virtio-auto test