KVM: selftests: Test disabling NX hugepages on a VM

Add an argument to the NX huge pages test to test disabling the feature
on a VM using the new capability.

Reviewed-by: David Matlack <dmatlack@google.com>
Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20220613212523.3436117-10-bgardon@google.com>
[Handle failure of sudo or setcap more gracefully. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>