Xen block driver now supports new featute called indirect descriptors.
This feature allows to put more data into each ring cell but it activates
for "long" reads and writes only - longer than 11 pages.

With this patch test by default runs 2 scenarious:
  * 1 page buffers
  * 32 pages buffers

Also introduced command line parameter to specify
size of buffers explicitly.

Signed-off-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>