librbd/migration: make ImageDispatch handle encryption for non-native formats

librbd/migration: make ImageDispatch handle encryption for non-native formats

With NativeFormat now being handled via dispatch, handling encryption
for non-native formats (i.e. mapping to raw image extents and performing
decryption/mapping back on completion) in the migration layer is really
straightforward.

Note that alignment doesn't need to be performed in the migration layer
because it happens on the destination image -- the "align and resubmit"
logic in C_UnalignedObjectReadRequest should kick in before the call to
read_parent().

Fixes: https://tracker.ceph.com/issues/53674
Co-authored-by: Or Ozeri <oro@il.ibm.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>