The various distros have the following libxml2 vesions:

        CentOS 7: 2.9.1
  Debian Stretch: 2.9.4
   FreeBSD Ports: 2.9.9
Ubuntu 16.04 LTS: 2.9.3

Based on this sampling, we can reasonably bump libxml2 min
version to 2.9.1

The 'query_raw' struct field was added in version 2.6.28,
so can be assumed to exist.
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>