Description of problem:
Power saving weight module not work in proper way for hosts with zero vms.

Version-Release number of selected component (if applicable):
rhevm-backend-3.6.0-0.12.master.el6.noarch

How reproducible:
Always

Steps to Reproduce:
1. Have two hosts with the same CPU arch(host_1, host_2)
2. Run vm with 1Gb on host_1
3. Change scheduler policy to power_saving with default parameters
4. Start additional vm

Actual results:
Vm start on host_2

Expected results:
Vm start on host_1

Additional info:
Problem that when host have zero vms, engine give to host score MaxSchedulerWeight - 1(999), but hosts with vms can have score bigger than(999) because expression:
"score -= vds.getMaxSchedulingMemory() / 100;" under calcEvenDistributionScore method in EvenDistributionWeightPolicyUnit class, we can have have negative score, and then under score method in PowerSavingWeightPolicyUnit class we run "score -= calcEvenDistributionScore(vds, vm, vdsGroup != null ? vdsGroup.getCountThreadsAsCores() : false);"
we can have have score bigger than MaxSchedulerWeight score.