zfs: remove snapshot dir on storage stop
zfs: remove snapshot dir on storage stop

This directory is created on zfs clone (it isn't used by the underlying zfs
snapshot) for snapshot storage start, so let's remove it on snapshot
storage stop.

Signed-off-by: Tycho Andersen <10c18fcb898ee6b17d6575d444b125d10b3f4d6a@canonical.com>

ollie314/lxd,georghopp/lxd,Skarlso/lxd,mjeanson/lxd,techtonik/lxd,hallyn/lxd,lxc/lxd,atxwebs/lxd,ollie314/lxd,akshaykarle/lxd,erikmack/lxd,yura-pakhuchiy/lxd,pcdummy/lxd,stgraber/lxd,hallyn/lxd,pcdummy/lxd,stgraber/lxd,stgraber/lxd,Skarlso/lxd,yura-pakhuchiy/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,techtonik/lxd,atxwebs/lxd,jsavikko/lxd,chrisglass/lxd,stgraber/lxd,stgraber/lxd,techtonik/lxd,erikmack/lxd,hallyn/lxd,lxc/lxd,georghopp/lxd,akshaykarle/lxd,atxwebs/lxd,Skarlso/lxd,jsavikko/lxd,dustinkirkland/lxd,lxc/lxd,mjeanson/lxd,yura-pakhuchiy/lxd,chrisglass/lxd,lxc/lxd,mjeanson/lxd,akshaykarle/lxd,hallyn/lxd,lxc/lxd,atxwebs/lxd,Skarlso/lxd,hallyn/lxd,dustinkirkland/lxd,pcdummy/lxd,georghopp/lxd,erikmack/lxd,hallyn/lxd,stgraber/lxd,chrisglass/lxd,ollie314/lxd,jsavikko/lxd,dustinkirkland/lxd