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>

LIBVIRT_ARG_WITH_ALT is more generic than LIBVIRT_ARG_WITH, which
is tailored at switching features on and off.

Rename the macros according to their intended purpose, and add
some documentation to help developers pick between the two.

Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

Pkg-config file libxml-2.0.pc was introduced in libxml2-2.2.4 and we
require at least 2.6.0 so the crazy xml2-config check can be dropped.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

Signed-off-by: NPavel Hrdina <phrdina@redhat.com>