make use of is_xml()
make use of is_xml()

We have a function is_xml, but it's not used anywhere. This patch
calls is_xml where appropriate.

Change-Id: I7909410f4e4db46837e618350da7c93bf31271ce

savinash47/openstack-doc-tools,openstack/openstack-doc-tools,openstack/openstack-doc-tools,savinash47/openstack-doc-tools,savinash47/openstack-doc-tools