lxd/storage/backend/lxd: Handle both old (incorrect) <parent>/<snap> and <snap> only names in index file in CreateCustomVolumeFromBackup
lxd/storage/backend/lxd: Handle both old (incorrect) <parent>/<snap> and <snap> only names in index file in CreateCustomVolumeFromBackup

Allows importing of both old and new custom volume backups with snapshots.

Fixes #9062

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

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