KVM: selftests: Add demand paging content to the demand paging test

The demand paging test is currently a simple page access test which, while
potentially useful, doesn't add much versus the existing dirty logging
test. To improve the demand paging test, add a basic userfaultfd demand
paging implementation.

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