engine: Use proper units in Storage QoS when talking to VDSM

The engine used Mbps and VDSM expects B/s. This patch changes
the engine side to use MB/s (as is more common when dealing with
storage) and properly translates that to Bytes when sending the
value to VDSM.

Change-Id: I2644e9e029c8b6ebe4b5b7626f7b1ed6b6b5aea4
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1145621
Signed-off-by: Martin Sivák <msivak@redhat.com>