OpenNebula

As reported in #3395

I tried to shrink an image (live running VM) and got: [one.vm.diskresize] New disk size has to be greater than current one. Please, let me shrink disks too. Just give a big "WARNING" message that you should already have shrunk filesytems / paritions, etc. ... but let me do it. If you think it is too "dangerous" for most use cases a oned option of "ALLOW_DISK_RESIZE_SHRINKING" could be used to enable this. Workloads change, ONE should be able to accomodate, whether capacity should increase or decrease.