<memballoon model='none'/> is the only way to disable balloon driver
since libvirt will add one automatically if the memballoon element is
missing. In other words, there's no balloon device if model is 'none'
and generating an alias for it makes no sense. The alias will be ignored
when parsing the XML and it will disappear once libvirtd is restarted.
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>