KVM: selftests: Add a helper to check EPT/VPID capabilities

Create a small helper function to check if a given EPT/VPID capability
is supported. This will be re-used in a follow-up commit to check for 1G
page support.

No functional change intended.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20220520233249.3776001-7-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>