qemu: plug child process leak on domain core dump

Detected by Coverity.  Leak introduced by typo in commit 58e668d2.

* src/qemu/qemu_driver.c (doCoreDump): Use correct function.

src/qemu/qemu_driver.c

@@ -2717,7 +2717,7 @@ doCoreDump(struct qemud_driver *driver,
 
 cleanup:
     VIR_FORCE_CLOSE(fd);
-    virFileDirectFdClose(directFd);
+    virFileDirectFdFree(directFd);
     if (ret != 0)
         unlink(path);
     return ret;