lxd/storage: Assume default size for block volumes when size is not set

This helps get correct VM disk total size.

Signed-off-by: hamistao <pedro.ribeiro@canonical.com>

lxd/storage/backend_lxd.go

@@ -3278,7 +3278,12 @@ func (b *lxdBackend) GetInstanceUsage(inst instance.Instance) (*VolumeUsage, err
 	}
 
 	sizeStr, ok := rootDiskConf["size"]
-	if ok {
+	if !ok && vol.ContentType() == drivers.ContentTypeBlock {
+		// If size config key is not set, assume default size for block volumes.
+		sizeStr = drivers.DefaultBlockSize
+	}
+
+	if sizeStr != "" {
 		total, err := units.ParseByteSizeString(sizeStr)
 		if err != nil {
 			return nil, err