KVM: selftests: Call a dummy helper in VM/vCPU ioctls() to enforce type

Replace the goofy static_assert on the size of the @vm/@vcpu parameters
with a call to a dummy helper, i.e. let the compiler naturally complain
about an incompatible type instead of homebrewing a poor replacement.

Reported-by: Andrew Jones <drjones@redhat.com>
Fixes: fcba483e ("KVM: selftests: Sanity check input to ioctls() at build time")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220613161942.1586791-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>