Fix kvm issues for Trusty
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 <021fe6d8989d7bdb08c8700d8c426ea80a140097@inktank.com>

dreamhost/teuthology,dmick/teuthology,ceph/teuthology,dmick/teuthology,caibo2014/teuthology,ivotron/teuthology,ivotron/teuthology,michaelsevilla/teuthology,SUSE/teuthology,t-miyamae/teuthology,ktdreyer/teuthology,zhouyuan/teuthology,dmick/teuthology,yghannam/teuthology,michaelsevilla/teuthology,tchaikov/teuthology,SUSE/teuthology,t-miyamae/teuthology,zhouyuan/teuthology,robbat2/teuthology,caibo2014/teuthology,ktdreyer/teuthology,SUSE/teuthology,tchaikov/teuthology,yghannam/teuthology,robbat2/teuthology,ceph/teuthology,dreamhost/teuthology