Remove remaining Arc<Volume> instances

[mkeeter]

Nested volumes use an Arc<dyn BlockIO + Send + Sync> to represent the inner BlockIO device.

Previously, we implemented the BlockIO trait on the struct Volume itself, so we needed to make an Arc<Volume> to create nested volumes.

This PR moves the BlockIO implementation to the VolumeInner. Because the VolumeInner is already stored in an Arc, we can now cast from Arc<VolumeInner> to Arc<dyn BlockIO + Send + Sync>, removing the extra level of indirection.

[leftwo]

Approved assuming there was no change to volume_write_op(), it just moved locations.

[mkeeter]

Approved assuming there was no change to volume_write_op(), it just moved locations.

That's correct!