tools: Add validation to MAC Address range

The patch adds a validation to the configured value of
the MAC Address pool range by the engine-config tool.

Change-Id: I821290d8d48ed164a5c653367d59774bd97bf67e
Bug-Url: https://bugzilla.redhat.com/947474
Signed-off-by: Moti Asayag <masayag@redhat.com>