tests: kvm: Add tests for KVM_SET_NESTED_STATE

Add tests for KVM_SET_NESTED_STATE and for various code paths in its implementation in vmx_set_nested_state().

Signed-off-by: Aaron Lewis <aaronlewis@google.com>
Reviewed-by: Marc Orr <marcorr@google.com>
Reviewed-by: Peter Shier <pshier@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>