Add override variable for LIBVIRT_TYPE

Allow jobs to set LIBVIRT_TYPE, but retain the current default.  This
is for testing nested virt.

Change-Id: Ife215db3f59f011574a50b0d1cbd5565a7408dfe

Author: ianw

.zuul.yaml

@@ -219,8 +219,8 @@
         VERBOSE_NO_TIMESTAMP: true
         NOVNC_FROM_PACKAGE: true
         ERROR_ON_CLONE: true
-        # Gate jobs can't deal with nested virt. Disable it.
-        LIBVIRT_TYPE: qemu
+        # Gate jobs can't deal with nested virt. Disable it by default.
+        LIBVIRT_TYPE: '{{ devstack_libvirt_type | default("qemu") }}'
       devstack_services:
         # Ignore any default set by devstack. Emit a "disable_all_services".
         base: false