Since 0ebd24e0, cdrom doesn't have read-only on by default, which will
error out when using an read only image. Fix it by setting the default
value when parsing opts.

Reported-by: Edivaldo de Araujo Pereira <edivaldoapereira@yahoo.com.br>
Signed-off-by: Fam Zheng <famz@redhat.com>

Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>