Fix kvm issues for Trusty

Change kvm reference to qemu-system-x86_64 and use raw format
in kvm/qemu command.  Tested on both Trusty and Precise.

Fixes: 7825
Signed-off-by: Warren Usui <warren.usui@inktank.com>
(cherry picked from commit eef2bf672b76a6c0243687c7acd20fd99e7dd44a)