KVM: selftests: Extend cpuid_test to verify KVM_GET_CPUID2 "nent" updates

Verify that KVM reports the actual number of CPUID entries on success, but
doesn't touch the userspace struct on failure (which for better or worse,
is KVM's ABI).

Link: https://lore.kernel.org/r/20230526210340.2799158-3-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>