Make kubevirt VM template configurable from values.yaml

Outcome from: !1713 (comment 2169605287)

Currently, the KubeVirt VM definitions under kubevirt-test-vms unit has hardcoded values for CPU, memory and other key VM settings.

This issue aims to make the VM template fully configurable through values.yaml, improving flexibility across different environments.

More parameters can be added as configurable as the need will arise.

cc: @feleouet