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 <jasowang@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20210315163450.254396-13-sgarzare@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>