engine: Treat bonds and VLANs properly in host network usage

Previously VLANs and bond slaves would have been included in the
search for the "bottleneck" interface in network usage, however VLAN
devices' speed is meaningless and bond slaves should be ignored (as
the bond itself will be evaluated).

Change-Id: I71adf32f76414fe34d433177741e73529959992b
Bug-Url: https://bugzilla.redhat.com/1114085
Signed-off-by: Lior Vernia <lvernia@redhat.com>