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>
(cherry picked from commit 824da2029613a6f4b380b6b2f16a0bd0903f7e3c)
Support latest qemu iotest code

Modified qemu-iotests workunit script to check for versions
that use the latest qemu (currently only Trusty).  Limit the
tests to those that are applicable to rbd.

Fixes: 7882
Signed-off-by: Warren Usui <warren.usui@inktank.com>
(cherry picked from commit 606e725eb5204e76e602d26ffd113e40af2ee812)
Use new git mirror for qemu-iotests

Fixes: 8191
Signed-off-by: Warren Usui <warren.usui@inktank.com>
(cherry picked from commit ddf37d903f826f3e153d8009c716780453b68b05)