fix copySparse for empty qcow2 disk

Signed-off-by: Nikita Vasilchenko <n@mintscale.ru>

pkg/nativeimgutil/nativeimgutil.go

@@ -127,6 +127,7 @@ func copySparse(w *os.File, r io.Reader, bufSize int64) (int64, error) {
 	zeroBuf := make([]byte, bufSize)
 	buf := make([]byte, bufSize)
 	var eof bool
+	hasWrites := false
 	for !eof {
 		rN, rErr := r.Read(buf)
 		if rErr != nil {
@@ -143,6 +144,7 @@ func copySparse(w *os.File, r io.Reader, bufSize int64) (int64, error) {
 			// no need to ftruncate here
 			n += int64(rN)
 		} else {
+			hasWrites = true
 			wN, wErr := w.Write(buf)
 			if wN > 0 {
 				n += int64(wN)
@@ -155,6 +157,11 @@ func copySparse(w *os.File, r io.Reader, bufSize int64) (int64, error) {
 			}
 		}
 	}
+
+	// Ftruncate must be run if the file contains only zeros
+	if !hasWrites {
+		return n, MakeSparse(w, n)
+	}
 	return n, nil
 }