sheepdog: Fix error handling in sd_snapshot_delete()

As a bdrv_snapshot_delete() method, sd_snapshot_delete() must set an
error and return negative errno on failure.  It sometimes returns -1,
and sometimes neglects to set an error.  It also prints error messages
with error_report().  Fix all that.

Moreover, its handling of an attempt to delete a nonexistent snapshot
is wrong: it error_report()s and succeeds.  Fix it to set an error and
return -ENOENT instead.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
