Instead of always using controller 0 and incrementing port number,
respect the maximum port numbers of controllers and use all of them.

Ports for virtio consoles are quietly reserved, but not formatted
(neither in XML nor on QEMU command line).

Also rejects duplicate virtio-serial addresses.
https://bugzilla.redhat.com/show_bug.cgi?id=890606
https://bugzilla.redhat.com/show_bug.cgi?id=1076708

Test changes:
* virtio-auto.args
  Filling out the port when just the controller is specified.
  switched from using
    maxport + 1
  to:
    first free port on the controller
* virtio-autoassign.args
  Filling out the address when no <address> is specified.
  Started using all the controllers instead of 0, also discards
  the bus value.
* xml -> xml output of virtio-auto
  The port assignment is no longer done as a part of XML parsing,
  so the unspecified values stay 0.