lxd/storage/drivers/utils: Updates ensureVolumeBlockFile to return unsupported when trying to resize image volume without allowUnsafeResize enabled
lxd/storage/drivers/utils: Updates ensureVolumeBlockFile to return unsupported when trying to resize image volume without allowUnsafeResize enabled

This is so the correct error response is returned to trigger backendLXD to regenerate cached BTRFS image volume when pool's volume.size changes.

Doesn't affect dir pools (which also use this function) as they do not have cached image volumes.

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

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