vdpa_sim_blk: implement ramdisk behaviour

vdpa_sim_blk: implement ramdisk behaviour

The previous implementation wrote only the status of each request.
This patch implements a more accurate block device simulator,
providing a ramdisk-like behavior and adding input validation.

Acked-by: Jason Wang <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Stefano Garzarella <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Michael S. Tsirkin <[email protected]>