KVM: selftests: Enable USERFAULTFD

The page_fault_test KVM selftest requires userfaultfd but the config
fragment for the KVM selftests does not enable it, meaning that those tests
are skipped in CI systems that rely on appropriate settings in the config
fragments except on S/390 which happens to have it in defconfig. Enable
the option in the config fragment so that the tests get run.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20230202-kvm-selftest-userfaultfd-v1-1-8186ac5a33a5@kernel.org


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>