libxl: fix fd check in libxl__spawn_local_dm

Checking the logfile_w fd for -1 on failure is no longer true, because
libxl__create_qemu_logfile will now return ERROR_FAIL on failure which
is -3.

While there also add an error check for opening /dev/null.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <Ian.Jackson@eu.citrix.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
(cherry picked from commit 3b88d95e9c)

Conflicts:
	tools/libxl/libxl_dm.c

tools/libxl/libxl_dm.c

@@ -1180,6 +1180,11 @@ void libxl__spawn_local_dm(libxl__egc *egc, libxl__dm_spawn_state *dmss)
     logfile_w = open(logfile, O_WRONLY|O_CREAT|O_APPEND, 0644);
     free(logfile);
     null = open("/dev/null", O_RDONLY);
+    if (null < 0) {
+        LOGE(ERROR, "unable to open /dev/null");
+        rc = ERROR_FAIL;
+        goto out_close;
+    }
 
     const char *dom_path = libxl__xs_get_dompath(gc, domid);
     spawn->pidpath = GCSPRINTF("%s/%s", dom_path, "image/device-model-pid");
@@ -1227,8 +1232,8 @@ retry_transaction:
     rc = 0;
 
 out_close:
-    close(null);
-    close(logfile_w);
+    if (null >= 0) close(null);
+    if (logfile_w >= 0) close(logfile_w);
 out:
     if (rc)
         device_model_spawn_outcome(egc, dmss, rc);