util: storage: Add helper to determine whether storage is local

util: storage: Add helper to determine whether storage is local

There's a lot of places where we skip doing actions based on the
locality of given storage type. The usual pattern is to skip it if:

virStorageSourceGetActualType(src) == VIR_STORAGE_TYPE_NETWORK

Add a simple helper to simplify the pattern to
virStorageSourceIsLocalStorage(src)