zfs: existence check before custom delete
zfs: existence check before custom delete

Closes #3775.

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@ubuntu.com>

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