Expose the virtio parameter for packed virtqueues as an optional libvirt
XML attribute to virtio-backed devices, e.g.:

    <interface type='user'>
      <mac address='00:11:22:33:44:55'/>
      <model type='virtio'/>
      <driver packed='on'/>
    </interface>

If the attribute is omitted, the default value for this attribute is 'off' and
regular split virtqueues are used.
Reviewed-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NBoris Fiuczynski <fiuczy@linux.ibm.com>
Signed-off-by: NBjoern Walk <bwalk@linux.ibm.com>
Signed-off-by: NJán Tomko <jtomko@redhat.com>