
When using file-backed storage, or SSD storage, any erasing done in the VM does not actually clear up the space. By using the virtio-scsi controller the VM is able to use TRIM to clear any blocks which are deleted. This also allows us to use fstrim to reduce the size of the qemu files before we save them for later re-use. Change-Id: Ia9001522ce054ee9f8a6dd38270da3e3fd039813