zfs: Don't fail on missing image snapshot
zfs: Don't fail on missing image snapshot

Closes #4188

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

lxc/lxd,pcdummy/lxd,Skarlso/lxd,erikmack/lxd,hallyn/lxd,mjeanson/lxd,jsavikko/lxd,stgraber/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,atxwebs/lxd,erikmack/lxd,atxwebs/lxd,Skarlso/lxd,mjeanson/lxd,jsavikko/lxd,stgraber/lxd,hallyn/lxd,hallyn/lxd,stgraber/lxd,hallyn/lxd,stgraber/lxd,lxc/lxd,atxwebs/lxd,pcdummy/lxd,lxc/lxd,erikmack/lxd,Skarlso/lxd,pcdummy/lxd,atxwebs/lxd,hallyn/lxd,stgraber/lxd,mjeanson/lxd,lxc/lxd,Skarlso/lxd,lxc/lxd,jsavikko/lxd,hallyn/lxd