The image now has enough mojo to run xts, hooray!

Note there's a couple of ugly bits in here. The piglit config fragment
and stock tetexec.cfg reflect missing features from the test suites
(using an installed copy of xts, figuring out the xserver test matrix at
runtime, etc.), I'll be happy to remove them once the tests are fixed.

Signed-off-by: Adam Jackson <ajax@redhat.com>