[libvirt] libvirt-tck failures after sparse stream support

[Jim Fehlig]

Hi Michal,

I noticed some of the libvirt-tck tests fail with 3.4.0 RC2. I bisected the 
failure to commit 07c2399c01. The failing tests are 400-vol-download.t, 
405-vol-download-all.t, and 410-vol-download-nonblock.t. Looking at the tests, 
they all create a pool and vol, write some data to the vol, create an MD5 digest 
of a subset of vol data, download the subset of the vol, and finally compare MD5 
digest of dowloaded vol vs original. If the digests are different the test 
fails. For reference, here is the first test

http://libvirt.org/git/?p=libvirt-tck.git;a=blob;f=scripts/storage/400-vol-download.t;h=2eb42b246b51b6dfc770f454fc1aea14d859fa7d;hb=HEAD

All three tests randomly fail with commit 07c2399c01. When the tests fail, it is 
always the case that 'length' was > 0. I.e. I have not seen the tests fail with 
offset=0, length=0 or offset=1024*1024, length=0.

Regards,
Jim