network_conf: Forbid commas in DNS TXT

network_conf: Forbid commas in DNS TXT

https://bugzilla.redhat.com/show_bug.cgi?id=1151942

While the restriction doesn't have origin in any RFC, it matters
to us while constructing the dnsmasq config file (or command line
previously). For better picture, this is how the corresponding
part of network XML look like:

  <dns>
    <forwarder addr='8.8.4.4'/>
    <txt name='example' value='example value'/>
  </dns>

And this is how the config file looks like then:

  server=8.8.4.4
  txt-record=example,example value

Now we can see why there can't be any commas in the TXT name.
They are used by dnsmasq to separate @name and @value.

Funny, we have it in the documentation, but the code (which was
pushed back in 2011) didn't reflect that.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>