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>
