Description of problem:
Currently, vm cloning job does not verify if the target VM already exists on a cluster.

> $ oc get vm
> NAME AGE STATUS READY
> vm-fedora 17m Running True
> vm-fedora2 13m Running True

VirtualMachineClone stuck in `VirtualMachineClone` phase:

> $ oc get vmclone
> NAME PHASE SOURCEVIRTUALMACHINE TARGETVIRTUALMACHINE
> clone-to-existing-vm RestoreInProgress vm-fedora vm-fedora2

> Normal SnapshotCreated 5m30s clone-controller created snapshot clone-clone-to-existing-vm-snapshot-vm-fedora-m2grh for clone clone-to-existing-vm
> Normal SnapshotReady 5m28s clone-controller snapshot clone-clone-to-existing-vm-snapshot-vm-fedora-m2grh for clone clone-to-existing-vm is ready to use

Also, it is possible to create cloning job with the same source/target VM:

> $ oc get vmclone
> NAME PHASE SOURCEVIRTUALMACHINE TARGETVIRTUALMACHINE
> clone-to-itself RestoreInProgress vm-fedora vm-fedora

Version-Release number of selected component (if applicable):
4.14

How reproducible:
100%

Steps to Reproduce:
1. create 2 VMs
2. try to clone the first VM with the target pointed to second VM
3.

Actual results:
VirtualMachineClone created and stuck in `VirtualMachineClone` phase

Expected results:
Seems, we need to have a validation and prevent creation of vmclone jobs if the target already exists

Additional info: