bugfix: Graceful shutdown of VMs

bugfix: Graceful shutdown of VMs

Cleanup phase of PVVP scenario sometimes causes server reboot.
It seems, that forced kill of VMs is the root cause. So graceful
shutdown of VMs was introduced.

Change-Id: I427404406fd7174d2a034f3cf0b51fe0833f9ecf
JIRA: VSPERF-271
Signed-off-by: Martin Klozik <martinx.klozik@intel.com>
Reviewed-by: Maryam Tahhan <maryam.tahhan@intel.com>
Reviewed-by: Al Morton <acmorton@att.com>
Reviewed-by: Christian Trautman <ctrautma@redhat.com>
Reviewed-by: Brian Castelli <brian.castelli@spirent.com>