block: add VIRTIO_BLK_F_BLK_SIZE feature

Now virtio-block devices always advertise VIRTIO_BLK_F_BLK_SIZE feature.
Users can specify new `blk_size` option for the block device to set this
value. If option is not set, default to 512 byte.

Signed-off-by: Egor Lazarchuk <yegorlz@amazon.co.uk>