sheepdog: fix snapshot tag initialization

This is an old and obvious bug. We should pass snapshot_id to the
tag. Or simple command like 'qemu-img snapshot -a tag sheepdog:image' will fail

Cc: qemu-devel@nongnu.org
Cc: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NLiu Yuan <namei.unix@gmail.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/sheepdog.c

@@ -2063,7 +2063,7 @@ static int sd_snapshot_goto(BlockDriverState *bs, const char *snapshot_id)
     if (snapid) {
         tag[0] = 0;
     } else {
-        pstrcpy(tag, sizeof(tag), s->name);
+        pstrcpy(tag, sizeof(tag), snapshot_id);
     }
 
     ret = reload_inode(s, snapid, tag);