Fix detached migration with exec.

When trying to do detached migration with exec, I found that
the monitor wouldn't always return in a timely manner.  I
tracked this down to exec_start_outgoing_migration.  It
appeared we were setting the fd to NONBLOCK'ing, but in
point of fact we weren't.

This bugfix should also go onto the stable 0.10 branch

Signed-off-by: Chris Lalancette <clalance@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Glauber Costa <glommer@redhat.com>

migration-exec.c

@@ -74,10 +74,7 @@ MigrationState *exec_start_outgoing_migration(const char *command,
         goto err_after_open;
     }
 
-    if (fcntl(s->fd, F_SETFD, O_NONBLOCK) == -1) {
-        dprintf("Unable to set nonblocking mode on file descriptor\n");
-        goto err_after_open;
-    }
+    socket_set_nonblock(s->fd);
 
     s->opaque = qemu_popen(f, "w");