qemu: Don't build cache= cmd line for scsi-block
    
    Trying to set any cache for <disk device='lun'/> makes no sense.
    Such disk translates into -device scsi-block on the command line
    and the device lacks any cache setting because it's merely a
    middle man between qemu and real SCSI device.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>