agent: Migrate engine VM only for local maintenance

Migrate the engine VM only when local maintenance is enabled and not
for the host score differentials.  This makes migration occur only when
it is likely to succeed.

Change-Id: I8fbf2e9c7eec0ca7787e71c6c328f95b243840c9
Bug-Url: https://bugzilla.redhat.com/1039614
Signed-off-by: Greg Padgett <gpadgett@redhat.com>