LXC: free dst before lxcDomainAttachDeviceDiskLive returns

Free dst before lxcDomainAttachDeviceDiskLive returns
Signed-off-by: NChen Hanxiao <chenhanxiao@cn.fujitsu.com>
(cherry picked from commit c82513ac)

src/lxc/lxc_driver.c

@@ -3177,6 +3177,7 @@ cleanup:
     virDomainAuditDisk(vm, NULL, def->src, "attach", ret == 0);
     if (dst && created && ret < 0)
         unlink(dst);
+    VIR_FREE(dst);
     return ret;
 }