KVM: selftests: Add a testcase to verify GUEST_MEMFD and READONLY are exclusive

Extend set_memory_region_test's invalid flags subtest to verify that
GUEST_MEMFD is incompatible with READONLY.  GUEST_MEMFD doesn't currently
support writes from userspace and KVM doesn't support emulated MMIO on
private accesses, and so KVM is supposed to reject the GUEST_MEMFD+READONLY
in order to avoid configuration that KVM can't support.

Link: https://lore.kernel.org/r/20240222190612.2942589-6-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/set_memory_region_test.c

@@ -375,6 +375,12 @@ static void test_invalid_memory_region_flags(void)
 		TEST_ASSERT(r && errno == EINVAL,
 			    "KVM_SET_USER_MEMORY_REGION2 should have failed, dirty logging private memory is unsupported");
 
+		r = __vm_set_user_memory_region2(vm, 0,
+						 KVM_MEM_READONLY | KVM_MEM_GUEST_MEMFD,
+						 0, MEM_REGION_SIZE, NULL, guest_memfd, 0);
+		TEST_ASSERT(r && errno == EINVAL,
+			    "KVM_SET_USER_MEMORY_REGION2 should have failed, read-only GUEST_MEMFD memslots are unsupported");
+
 		close(guest_memfd);
 	}
 }