Fix uninitialized value in QEMU monitor FD sending code

The 'char control[CMSG_SPACE(sizeof(int))];' was not being
wiped, so could potentially contain uninitialized bytes.
While this was harmless in this case, it caused complaints
from valgrind

* src/qemu/qemu_monitor.c: memset 'control' variable
  in qemuMonitorIOWriteWithFD

src/qemu/qemu_monitor.c

@@ -383,6 +383,7 @@ qemuMonitorIOWriteWithFD(qemuMonitorPtr mon,
     struct cmsghdr *cmsg;
 
     memset(&msg, 0, sizeof(msg));
+    memset(control, 0, sizeof(control));
 
     iov[0].iov_base = (void *)data;
     iov[0].iov_len = len;