lxd/storage/backend/lxd: Update RefreshCustomVolume to use srcPool.GenerateCustomVolumeBackupConfig
lxd/storage/backend/lxd: Update RefreshCustomVolume to use srcPool.GenerateCustomVolumeBackupConfig

And to support migration index header.
Also reworks function to avoid unnecessary intermediate variables.

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

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