Support latest qemu iotest code

Modified qemu-iotests workunit script to check for versions
that use the latest qemu (currently only Trusty).  Limit the
tests to those that are applicable to rbd.

Fixes: 7882
Signed-off-by: Warren Usui <warren.usui@inktank.com>
Merge pull request #1718 from ceph/wip-7882-wusui

Support latest qemu iotest code

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>