Description of problem:
Common template rootDisk is not named correctly, when creating a VM from common template the rootDisk is named by the VM name instead of rootDisk
this is a problem because:
a. the convention in common templates is to call the disk by it's use, e.g. "cloudinitdisk", "default" and not by the vm name
b. it is not matching the anotation "defaults.template.kubevirt.io/disk"
c. it is making harder on automation utilities to finding the rootdisk

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

How reproducible:

Steps to Reproduce:
1. create a virtual machine from a common template
2. look for a disk matching the name in defaults.template.kubevirt.io/disk anotation
3. look for a "rootDisk" disk

Actual results:
no disk matching the annotation found (for use cases looking by annotation)
no "rootDisk" disk found (for use cases looking by disk usage)

Expected results:
a "rootDisk" is found the the "defaults.template.kubevirt.io/disk" annotation is pointing at it

Additional info: