KVM: selftests: Add NX huge pages test

There's currently no test coverage of NX hugepages in KVM selftests, so
add a basic test to ensure that the feature works as intended.

The test creates a VM with a data slot backed with huge pages. The
memory in the data slot is filled with op-codes for the return
instruction. The guest then executes a series of accesses on the memory,
some reads, some instruction fetches. After each operation, the guest
exits and the test performs some checks on the backing page counts to
ensure that NX page splitting an reclaim work as expected.

Reviewed-by: David Matlack <dmatlack@google.com>
Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20220613212523.3436117-7-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>