tests: add virtio-blk discard integration coverage

Cover the discard drive API by validating successful configuration,
guest-visible discard granularity, and rejection of unsupported
read-only or async combinations.

Add a guest functional test that writes data to a file-backed drive,
issues blkdiscard, and verifies host block allocation decreases.

[jackabt@amazon.com: rebased onto main]

Signed-off-by: Jonas Savulionis <jonas@esnet.lt>
Signed-off-by: Rekas <aure369@gmail.com>
Signed-off-by: Jack Thomson <jackabt@amazon.com>
Signed-off-by: Pierre Bertholom <pbertho@amazon.com>