lxd/storage/drivers/driver/btrfs/utils: Adds marshal tags to BTRFSSubVolume and BTRFSMetaDataHeader
lxd/storage/drivers/driver/btrfs/utils: Adds marshal tags to BTRFSSubVolume and BTRFSMetaDataHeader

Indicates why they are exported types and helps with future plans to encode this into a yaml file inside backup files.

Backwards compatible with old untagged format.

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

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