sheepdog: mark image as snapshot when tag is specified

When a snapshot tag is specified in the filename, the opened image is
a snapshot.
Signed-off-by: NMORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/sheepdog.c

@@ -1103,7 +1103,7 @@ static int sd_open(BlockDriverState *bs, const char *filename, int flags)
         }
     }
 
-    if (snapid) {
+    if (snapid || tag[0] != '\0') {
         dprintf("%" PRIx32 " snapshot inode was open.\n", vid);
         s->is_snapshot = 1;
     }