ahci-test: improve rw buffer patterns

My pattern was cyclical every 256 bytes, so it missed a fairly obvious
failure case. Add some rand() pepper into the test pattern, and for large
patterns that exceed 256 sectors, start writing an ID per-sector so that
we never generate identical sector patterns.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Tested-by: Andreas Färber <afaerber@suse.de>
Message-id: 1426811056-2202-5-git-send-email-jsnow@redhat.com