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>
Merge pull request #1575 from jdurgin/wip-librbd-cp-sparse

librbd: skip zeroes when copying an image

Reviewed-by: Sage Weil <sage@inktank.com>