Given that the default is to use raw for device drives and qcow2 for
file drives,  and that 'discard' was already enabled for raw, this
commit is in practice mainly for file drives, for which QEMU processes
TRIM by punching holes (i.e. the image file becomes sparse).

This reduces disk usage not only by the process running the VM but
also by backups and replicas (e.g. runner1).

For qcow2 at least, the discarded space can be reused for other blocks
and qemu compacts the image at dump (for backups) so images on replicas
are not sparse.

When an image disk format doesn't support it, QEMU ignores the option
so it's safe.

Guest support of TRIM for virtio-blk is recent: Linux 5.0