drivers: Make block device init silent by default

We now print disk probe information like:

virtio-scsi: Add scsi device target=1, lun=0   as vblk0, devsize=10842275840

Disable it by default. The --verbose option will make this info
available which is useful when debug.

Reported-by: Pekka Enberg <penberg@cloudius-systems.com>
Signed-off-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>