lxd/migrate/storage/volumes: Add support for migration index frame for custom volume migration
lxd/migrate/storage/volumes: Add support for migration index frame for custom volume migration

 - Pass volume config to MigrateCustomVolume in migrationSourceWs.DoStorage
 - This fixes a bug that was preventing a source from successfully mounting the volume to send if there was specific mount config needed.
 - Adds logic to prune the included snapshot config based on the snapshots being requested during refresh.
 - Adds migration negotiation of the supported index header frame version in migrationSourceWs and migrationSink DoStorage().

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

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