core: loadbalancing: prevent VM bounces between hosts

When balancing a cluster, the balancing logic should return
a VM and acceptable hosts, while filtering those hosts
the scheduler#balance doesn't take into account the predicted VM CPU load.
This may result in a VM bouncing between 2 hosts (if VM consumes
all the CPU resources on both hosts).

Change-Id: I6beebf4324bfa5218d9617619b350fb9a2de56d3
Bug-Url: https://bugzilla.redhat.com/1043233
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>