core: Check that balance function choose source host with at least one vm

Now balance function of PowerSaving or EvenlyDistributed
cluster policy can choose migration source host without vm,
just relying on cpu loading and it create delay in time of balancing.

Change-Id: I448f9b201308599d9d60e27c2464b586129f3cca
Signed-off-by: Artyom Lukianov <alukiano@redhat.com>