Split out storage format 'compat' attribute sanity check

For future reuse in the snapshot XML.

src/conf/storage_conf.c

@@ -1374,20 +1374,8 @@ virStorageVolDefParseXML(virStoragePoolDefPtr pool,
     }
 
     ret->target.compat = virXPathString("string(./target/compat)", ctxt);
-    if (ret->target.compat) {
-        char **version = virStringSplit(ret->target.compat, ".", 2);
-        unsigned int result;
-
-        if (!version || !version[1] ||
-            virStrToLong_ui(version[0], NULL, 10, &result) < 0 ||
-            virStrToLong_ui(version[1], NULL, 10, &result) < 0) {
-            virStringFreeList(version);
-            virReportError(VIR_ERR_XML_ERROR, "%s",
-                           _("forbidden characters in 'compat' attribute"));
+    if (virStorageFileCheckCompat(ret->target.compat) < 0)
         goto error;
-        }
-        virStringFreeList(version);
-    }
 
     if (virXPathNode("./target/nocow", ctxt))
         ret->target.nocow = true;

src/util/virstoragefile.c

@@ -2873,3 +2873,32 @@ virStorageFileGetRelativeBackingPath(virStorageSourcePtr top,
     VIR_FREE(tmp);
     return ret;
 }
+
+
+/*
+ * virStorageFileCheckCompat
+ */
+int
+virStorageFileCheckCompat(const char *compat)
+{
+    char **version;
+    unsigned int result;
+    int ret = -1;
+
+    if (!compat)
+        return 0;
+
+    version = virStringSplit(compat, ".", 2);
+    if (!version || !version[1] ||
+        virStrToLong_ui(version[0], NULL, 10, &result) < 0 ||
+        virStrToLong_ui(version[1], NULL, 10, &result) < 0) {
+        virReportError(VIR_ERR_XML_ERROR, "%s",
+                       _("forbidden characters in 'compat' attribute"));
+        goto cleanup;
+    }
+    ret = 0;
+
+ cleanup:
+    virStringFreeList(version);
+    return ret;
+}

src/util/virstoragefile.h

@@ -384,4 +384,5 @@ int virStorageFileGetRelativeBackingPath(virStorageSourcePtr from,
                                          char **relpath)
     ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(2) ATTRIBUTE_NONNULL(3);
 
+int virStorageFileCheckCompat(const char *compat);
 #endif /* __VIR_STORAGE_FILE_H__ */