virtio-blk: add discard config space fields

Extend the virtio-blk config space through the discard limit fields so
the device can report discard capability details when the feature is
enabled.

[jackabt@amazon.com: rebased onto main and reconciled with the
read-only config space test]

Signed-off-by: Jonas Savulionis <jonas@esnet.lt>
Signed-off-by: Jack Thomson <jackabt@amazon.com>