Add TXT record support for virtual DNS service

Add TXT record support for virtual DNS service

This commit introduces the <dns> element and <txt> record for the
virtual DNS network. The DNS TXT record can be defined using following
syntax in the network XML file:

  <dns>
    <txt name="example" value="example value" />
  </dns>

Also, the Relax-NG scheme has been altered to allow the texts without
spaces only for the name element and some nitpicks about memory
free'ing have been fixed by Laine so therefore I'm adding Laine to the
SOB clause ;-)

Signed-off-by: Michal Novotny <minovotn@redhat.com>
Signed-off-by: Laine Stump <laine@laine.org>