sheepdog: Fix misleading error messages in sd_snapshot_create()

If do_sd_create() fails, it first reports the error returned, then
reports a another one with strerror(errno).  errno is meaningless at
that point.

Report just one error combining the valid information from both
messages.
Reported-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NLiu Yuan <namei.unix@gmail.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/sheepdog.c

@@ -2339,9 +2339,8 @@ static int sd_snapshot_create(BlockDriverState *bs, QEMUSnapshotInfo *sn_info)
 
     ret = do_sd_create(s, &new_vid, 1, &local_err);
     if (ret < 0) {
-        error_report_err(local_err);
-        error_report("failed to create inode for snapshot. %s",
-                     strerror(errno));
+        error_report("failed to create inode for snapshot: %s",
+                     error_get_pretty(local_err));
         goto cleanup;
     }