vmnetfs: Drop unnecessary out-argument initialization

GError semantics say we don't have to set out-arguments on failure.
vmnetfs: Move copy of partial chunk on truncate into helper function
vmnetfs: Copy partial last chunk to modified cache during extend

Extending a partial last chunk is modifying that chunk, so we need to
copy it to the modified cache.  This ensures that the chunks_modified
bitmap is correct, and also fixes an assertion when extending a partial
last chunk via truncate(2) and then trying to read it:

    vmnetfs/ll-pristine.c:139:_vmnetfs_ll_pristine_read_chunk: assertion failed: (chunk * img->chunk_size + offset + length <= img->initial_size)
vmnetfs: Add stream enumerating chunk fetches
vmnetfs: Avoid chunk fetch when overwriting the entire chunk