selftests: kvm: do not set guest mode flag

Setting KVM_STATE_NESTED_GUEST_MODE enables various consistency checks
on VMCS12 and therefore causes KVM_SET_NESTED_STATE to fail spuriously
with -EINVAL.  Do not set the flag so that we're sure to cover the
conditions included by the test, and cover the case where VMCS12 is
set and KVM_SET_NESTED_STATE is called with invalid VMCS12 contents.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>