Commit bcada37b dropped the (up to now undocumented) members type, len,
offset, speed, breaking tests/qemu-iotests/040 and 041.

Restore and document them.  This fixes 040, and partially fixes 041.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Tested-By: NBenoit Canet <benoit@irqsave.net>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>