io: use memset instead of { 0 } for initializing array

Some versions of GCC on OS-X complain about CMSG_SPACE
not being constant size, which prevents use of { 0 }

io/channel-socket.c: In function 'qio_channel_socket_writev':
io/channel-socket.c:497:18: error: variable-sized object may not be initialized
     char control[CMSG_SPACE(sizeof(int) * SOCKET_MAX_FDS)] = { 0 };

The compiler is at fault here, but it is nicer to avoid
tickling this compiler bug by using memset instead.

Reviewed-By: John Arbuckle <programmingkidx@gmail.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

io/channel-socket.c

@@ -449,6 +449,8 @@ static ssize_t qio_channel_socket_readv(QIOChannel *ioc,
     char control[CMSG_SPACE(sizeof(int) * SOCKET_MAX_FDS)];
     int sflags = 0;
 
+    memset(control, 0, CMSG_SPACE(sizeof(int) * SOCKET_MAX_FDS));
+
 #ifdef MSG_CMSG_CLOEXEC
     sflags |= MSG_CMSG_CLOEXEC;
 #endif
@@ -493,10 +495,12 @@ static ssize_t qio_channel_socket_writev(QIOChannel *ioc,
     QIOChannelSocket *sioc = QIO_CHANNEL_SOCKET(ioc);
     ssize_t ret;
     struct msghdr msg = { NULL, };
-    char control[CMSG_SPACE(sizeof(int) * SOCKET_MAX_FDS)] = { 0 };
+    char control[CMSG_SPACE(sizeof(int) * SOCKET_MAX_FDS)];
     size_t fdsize = sizeof(int) * nfds;
     struct cmsghdr *cmsg;
 
+    memset(control, 0, CMSG_SPACE(sizeof(int) * SOCKET_MAX_FDS));
+
     msg.msg_iov = (struct iovec *)iov;
     msg.msg_iovlen = niov;