misc: Cleaner readspeed

When checking reading speed from storage, we used to actually read the
data from the dd process into vdsm, just to drop it on the floor. This
makes no sense since we are interested only in the 3 lines of statistics
read from dd stderr.

This patch uses execCmd directly instead of using the generic _readfile
and configure dd to write the data to /dev/null.

Profiling show no significant change in cpu usage, but doing less work
is always better than doing more work.

Change-Id: Ib6145d018a691321976d684a9f355e3eb5252f83
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27616
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>