storage: Extract common code to retrieve driver backend for support check

The 'file access' module of the storage driver has few feature checks to
determine whether libvirt supports given storage driver method. The code
to retrieve the driver struct needed for the check is the same so it can
be extracted.

src/storage/storage_source.c

@@ -44,24 +44,30 @@ virStorageFileIsInitialized(const virStorageSource *src)
 }
 
 
-static bool
-virStorageFileSupportsBackingChainTraversal(virStorageSourcePtr src)
+static virStorageFileBackendPtr
+virStorageFileGetBackendForSupportCheck(const virStorageSource *src)
 {
     int actualType;
-    virStorageFileBackendPtr backend;
 
     if (!src)
-        return false;
+        return NULL;
+
+    if (src->drv)
+        return src->drv->backend;
+
     actualType = virStorageSourceGetActualType(src);
 
-    if (src->drv) {
-        backend = src->drv->backend;
-    } else {
-        if (!(backend = virStorageFileBackendForTypeInternal(actualType,
-                                                             src->protocol,
-                                                             false)))
-            return false;
-    }
+    return virStorageFileBackendForTypeInternal(actualType, src->protocol, false);
+}
+
+
+static bool
+virStorageFileSupportsBackingChainTraversal(virStorageSourcePtr src)
+{
+    virStorageFileBackendPtr backend;
+
+    if (!(backend = virStorageFileGetBackendForSupportCheck(src)))
+        return false;
 
     return backend->storageFileGetUniqueIdentifier &&
            backend->storageFileRead &&
@@ -80,21 +86,10 @@ virStorageFileSupportsBackingChainTraversal(virStorageSourcePtr src)
 bool
 virStorageFileSupportsSecurityDriver(const virStorageSource *src)
 {
-    int actualType;
     virStorageFileBackendPtr backend;
 
-    if (!src)
+    if (!(backend = virStorageFileGetBackendForSupportCheck(src)))
         return false;
-    actualType = virStorageSourceGetActualType(src);
-
-    if (src->drv) {
-        backend = src->drv->backend;
-    } else {
-        if (!(backend = virStorageFileBackendForTypeInternal(actualType,
-                                                             src->protocol,
-                                                             false)))
-            return false;
-    }
 
     return !!backend->storageFileChown;
 }