To ensure that the device addressing scheme is stable across
hotplug/unplug, all virtio serial channels needs to have an
associated port number in their address. This is then specified
to QEMU using the nr=NNN parameter

* src/conf/domain_conf.c, src/conf/domain_conf.h: Parsing
  for port number in vioserial address types.
* src/qemu/qemu_conf.c: Set 'nr=NNN' parameter with virtio
  serial port number
* tests/qemuxml2argvdata/qemuxml2argv-channel-virtio.args,
  tests/qemuxml2argvdata/qemuxml2argv-channel-virtio.xml: Expand
  data set to ensure coverage of port addressing