Description of problem:
When trying to start a VM with VirtualMachinePreference with "clock" parameters the VM won't start giving an Error of:

Failure while starting VMI: VirtualMachineInstance.kubevirt.io "rhel-vm"
is invalid: spec.domain.clock.timer: Invalid value: "null": spec.domain.clock.timer in body must be of type object: "null"

Version-Release number of selected component (if applicable):
4.12

How reproducible:
100%

Steps to Reproduce:
1. Create a VirtualMachinePreference with "clock" value
2. Create a VM with reference to the preference
3. Start the VM

Actual results:
VM wont start

Expected results:
VM will start

Additional info:

• Adding to the VM an empty dict under such as - spec.template.spec.domain.clock = {}, will let the VM start

• Attached yaml files of run