storage: Check stderr when matching parted output

In old version of parted like parted-2.1-25, error message is shown in
stdout when printing a disk info without disk label.

    Error: /dev/sda: unrecognised disk label

This line has been moved to stderr in newer version of parted. So we
should check both stdout and stderr when locating this message.

This should fix bug:
    https://bugzilla.redhat.com/show_bug.cgi?id=1172468Signed-off-by: NHao Liu <hliu@redhat.com>

src/storage/storage_backend_disk.c

@@ -370,21 +370,26 @@ virStorageBackendDiskFindLabel(const char* device)
     };
     virCommandPtr cmd = virCommandNew(PARTED);
     char *output = NULL;
+    char *error = NULL;
     int ret = -1;
 
     virCommandAddArgSet(cmd, args);
     virCommandAddEnvString(cmd, "LC_ALL=C");
     virCommandSetOutputBuffer(cmd, &output);
+    virCommandSetErrorBuffer(cmd, &error);
 
     /* if parted succeeds we have a valid partition table */
     ret = virCommandRun(cmd, NULL);
     if (ret < 0) {
-        if (strstr(output, "unrecognised disk label"))
+        if (strstr(output, "unrecognised disk label") ||
+            strstr(error, "unrecognised disk label")) {
             ret = 1;
+        }
     }
 
     virCommandFree(cmd);
     VIR_FREE(output);
+    VIR_FREE(error);
     return ret;
 }