core: error in calculating number of available macs / trimming mac pool ranges

mac ranges was trimmed to inapproriate size; it was trimmed so that
first and last mac representation can 'fit' into int datatype, but
that rendered unable to fit *count* of macs in such range into int
datatype, since that count would then be max_int+1 and that'd cause
overflow.

Bug-Url: https://bugzilla.redhat.com/1162077
Change-Id: I4bbf719deb47c795862a03deb515283b1eae9a0e
Signed-off-by: Martin Mucha <mmucha@redhat.com>