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