librbd/io: ReadResult image-extent helper now takes a buffer offset

librbd/io: ReadResult image-extent helper now takes a buffer offset

All existing users will continue to pass a buffer offset of zero, but
a future HTTP-based transfer class will need to potentially break up
multiple extents into individual HTTP requests and therefore will need
to adjust the offset.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>