Fix auto-adding of virtio serial controllers

Fix auto-adding of virtio serial controllers

The domain parsing code would auto-add a virtio serial controller
if it saw any virtio serial channel defined. Unfortunately it
always added a controller with index=0, even if the channel address
specified an index != 0. It only added one controller, even if
multiple controllers were referenced by channels. Finally, it let
the ports+vectors parameters initialize to zero instead of -1, which
prevented the controllers accepting any ports.

* src/conf/domain_conf.c: Initialize ports+vectors when adding
  virtio serial controllers. Add all neccessary virtio serial
  controllers, instead of hardcoding controller 0
* qemuxml2argvdata/qemuxml2argv-channel-virtio.args,
  qemuxml2argvdata/qemuxml2argv-channel-virtio.xml: Expand to
  test controller auto-add behaviour