librados: fix list_snaps() tests

The ioctx must be set to SNAP_DIR, and the head does not include any
clones.

Bug #4623

Signed-off-by: David Zafman <david.zafman@inktank.com>
osd: fail with EINVAL if bad snapid set on list_snaps

Signed-off-by: David Zafman <david.zafman@inktank.com>
librados: set SNAP_DIR on listsnaps command

Signed-off-by: David Zafman <david.zafman@inktank.com>
librados: test for EINVAL on bad list_snaps() ioctx snapid

Signed-off-by: Sage Weil <sage@inktank.com>