lxd/storage/drivers/interface: Use Volume type for bucket functions
lxd/storage/drivers/interface: Use Volume type for bucket functions

In order to support local buckets we need to support storage pool volume settings.
So it makes sense to re-use the existing type.

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

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