Xen: restrict devstack VM use upto 8 vCPUs.
Xen: restrict devstack VM use upto 8 vCPUs.

Change-Id: Iea6086a3feb03dff581d9c04560133c4f57a6f6c
Closes-Bug: 1672642

group-policy/devstack,nawawi/openstack,dtroyer/devstack,openstack-dev/devstack,dtroyer/devstack,noironetworks/devstack,richliu/devstack,group-policy/devstack,noironetworks/devstack,nawawi/openstack,richliu/devstack,openstack-dev/devstack