Implement DNS SRV record into the bridge driver

Implement DNS SRV record into the bridge driver

Hi,
this is the fifth version of my SRV record for DNSMasq patch rebased
for the current codebase to the bridge driver and libvirt XML file to
include support for the SRV records in the DNS. The syntax is based on
DNSMasq man page and tests for both xml2xml and xml2argv were added as
well. There are some things written a better way in comparison with
version 4, mainly there's no hack in tests/networkxml2argvtest.c and
also the xPath context is changed to use a simpler query using the
virXPathInt() function relative to the current node.

Also, the patch is also fixing the networkxml2argv test to pass both
checks, i.e. both unit tests and also syntax check.

Please review,
Michal

Signed-off-by: Michal Novotny <minovotn@redhat.com>