[ovirt] fix issues with oVirt 3.1 API.

In version 3.1 of oVirt API there is a change in the meaning of the virtual machine (VM) status. When creating a VM in versions prior to 3.1, it used to be in status 'image-locked' until the virtual disks where ready. In version 3.1 a change in the API was introduced, the VM status no longer represent the status of the VM and it's disks, from this version, in order to know that a VM is ready for launch, we need to loop through the disks checking the state of each one.
This patch fix the oVirt launch logic, it is backward compatible with previous oVirt versions.