virt-libvirt: Accessor generators for common XML mods

Signed-off-by: Chris Evich <cevich@redhat.com>
virt-libvirt: Move libvirt_xml into a package

Signed-off-by: Chris Evich <cevich@redhat.com>
fixup! virt-libvirt: Move libvirt_xml into a package

Signed-off-by: Chris Evich <cevich@redhat.com>
virt-libvirt: Update libvirt_xml unittests

Signed-off-by: Chris Evich <cevich@redhat.com>