lxd/storage/backend/lxd: Restore pool directory structure on mount if needed
lxd/storage/backend/lxd: Restore pool directory structure on mount if needed

Allows for recovery of pools that don't have a mounted root.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

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