iotests: Add "add_drive_raw" method

This offers full manual control over the "-drive" options.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 1448962590-2842-3-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>