domain: use the dd statistics for the domain delay

The previous implementation of getReadDelay was calculating the reading
time (for the domain statistics) checking the execution time of dd.
This patch introduces a low level readspeed command to parse the more
accurate statistics provided by dd in the stderr.

Bug-Url: http://bugzilla.redhat.com/show_bug.cgi?id=948232
Change-Id: Ib3fa54dbbe3475b74bcfe5cbc0c60895818f8b3e
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/15049
Reviewed-by: Giuseppe Vallarelli <gvallare@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>