In case of 'sd' cards we'll use pre-blockdev code also if qemu supports
blockdev. In that specific case we'll need to mask out blockdev support
for 'sd' disks. Plumb in a boolean to allow it.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>