KVM: selftests: Create a demand paging test

While userfaultfd, KVM's demand paging implementation, is not specific
to KVM, having a benchmark for its performance will be useful for
guiding performance improvements to KVM. As a first step towards creating
a userfaultfd demand paging test, create a simple memory access test,
based on dirty_log_test.

Reviewed-by: Oliver Upton <oupton@google.com>
Signed-off-by: Ben Gardon <bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>