engine: Initialize MAC pool partially

If MAC pool exceeds the maximum then we will initialize it until its
full. An error is being written to the log in this case.

The partial initialization will prevent an OOM in case of a really big
range (not entirely though, this will be fixed in the future).

Change-Id: I0547509eb81a51599e2342503c9f108df1fa47c7
Bug-Url: https://bugzilla.redhat.com/1056803
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>