engine: Prevent empty MAC Address range

The patch prevents a user from entering a mac address
range which might result in an empty pool, for example
a range which contains multicast mac addresses only.

Change-Id: I24113379e8fbbc15a63bdb1be4e6c719d7cb764f
Bug-Url: https://bugzilla.redhat.com/1011912
Signed-off-by: Moti Asayag <masayag@redhat.com>