drives: expose 'readonly' flag of qemu for scsi/virtio

drives: expose 'readonly' flag of qemu for scsi/virtio

this allows a user to set a drive to 'read-only'. This can be useful
if a disk should not be written to, or if the backing file/source is
not writable (like a mapped pbs backup to /dev/loopX).

the option is named 'ro', to achieve consistency with containers

while this could also be achieved by setting 'snapshot=1', this would
create a temporary file in /var/tmp which can get quite big.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>