lxd/storage/driver/zfs: Fix ListVolumes with custom zpool
lxd/storage/driver/zfs: Fix ListVolumes with custom zpool

Closes #9207

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

hallyn/lxd,atxwebs/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd,atxwebs/lxd,lxc/lxd,atxwebs/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,atxwebs/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd