kvm tools: Fix kvm-ipc.c build breakage

      CC       kvm-ipc.o
    kvm-ipc.c: In function ???kvm_ipc__send_msg???:
    kvm-ipc.c:53:34: error: comparison between signed and unsigned integer
    expressions [-Werror=sign-compare]
    cc1: all warnings being treated as errors

Reported-by: Jean-Philippe Menil <jean-philippe.menil@univ-nantes.fr>
Signed-off-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>
diff --git a/kvm-ipc.c b/kvm-ipc.c
index dbe2363..5b65882 100644
--- a/kvm-ipc.c
+++ b/kvm-ipc.c
@@ -50,7 +50,7 @@
 	if (write_in_full(fd, &head, sizeof(head)) != sizeof(head))
 		return -1;
 
-	if (write_in_full(fd, msg, len) != len)
+	if (write_in_full(fd, msg, len) < 0)
 		return -1;
 
 	return 0;