lxd: Properly unmount old image volume
lxd: Properly unmount old image volume

This fixes a bug where the old image volume would not be unmounted
properly because the storage volume name is passed incorrectly.

Signed-off-by: Thomas Hipp <5f82c492b3b00e427412d216ce820707a10c51ce@canonical.com>

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