zfs: Only delete copy- snapshots on delete
zfs: Only delete copy- snapshots on delete

Closes #2127

Signed-off-by: Stéphane Graber <089afc6d81f66f1168a9849e15660feae286e024@ubuntu.com>

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