qtest/ide: add another short PRDT test flavor

The existing short PRDT test case does not transfer any data because the
first PRD is less than 1 sector.

This patch adds another short PRDT test case where the first sector can
be read but the PRDT is still smaller than the requested number of
sectors.  This exercises a different code path in ide_dma_cb().

Cc: John Snow <jsnow@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 1435770571-9906-1-git-send-email-stefanha@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>