librbd: skip zeroes when copying an image

This is the simple coarse-grained solution, but it works well in common cases
like a small base image resized with a bunch of empty space at the end.
Finer-grained sparseness can be copied by using rbd
{export,import}-diff.

Fixes: #6257 Signed-off-by: Josh Durgin <josh.durgin@inktank.com>