Add testcase which checks that allocations during copy-on-read are
performed on the subcluster basis when subclusters are enabled in target
image.
This testcase also triggers the following assert with previous commit
not being applied, so we check that as well:
qemu-io: ../block/io.c:1236: bdrv_co_do_copy_on_readv: Assertion `skip_bytes <
pnum' failed.
Reviewed-by: Eric Blake<eblake@redhat.com>
Reviewed-by: Denis V. Lunev<den@openvz.org>
Signed-off-by: Andrey Drobyshev<andrey.drobyshev@virtuozzo.com>