Support OpenGL when using SDL backend via -sdl,gl=on. Add associated
tests.

NB: Usage of DO_TEST_CAPS_LATEST in qemuxml2argv doesn't work in
this case because -sdl gl is not introspectable.
Signed-off-by: NMaciej Wolny <maciej.wolny@codethink.co.uk>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>