Bump stack.libvirt-vm role to 1.8.0

This version adds the ACPI feature [1] to the guest definition, which
should help to support graceful shutdowns with `virsh shutdown`.

[1] https://github.com/stackhpc/ansible-role-libvirt-vm/commit/8405c20cf9b0ce86295b431a0bb07f8814b03f70

Change-Id: I56533ead8357945a42b079fda7edbd9912fc135a

requirements.yml

@@ -26,7 +26,7 @@
 - src: stackhpc.libvirt-host
   version: v1.3.0
 - src: stackhpc.libvirt-vm
-  version: v1.6.0
+  version: v1.8.0
 - src: stackhpc.mellanox-switch
   version: v1.0.0
 - src: stackhpc.os-images