Print only xml files for --print-unused-files
Print only xml files for --print-unused-files

The command openstack-doc-tools --check-build --print-unused-files
only handles .xml files but no other dependencies, so output only
.xml files.

Change-Id: I6c39446cb38ca6e73e3b75597a4e38c49bb56ac7

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