Small XLA backend changes that would be nice to fix before the release

[mooskagh]

• Increase the backend priority so that it's above random
• Set has_backends to true so that meson doesn't complain when XLA is the only backend
• Maybe downgrade pjrt_c_api.h to something lower than 0.40 so that it runs with older .so files, but it seems that 0.39 is the minimal one that has all features that we need, so maybe not worth it.