CI: use meta-virtualization master-next branch

Use master-next for meta-virt while waiting for the syntax change
patches to make it into master

Signed-off-by: Jon Mason <jon.mason@arm.com>

ci/gem5-arm64-xen.yml

@@ -17,6 +17,8 @@ repos:
       meta-networking:
   meta-virtualization:
     url: git://git.yoctoproject.org/meta-virtualization
+    #FIXME - remove after master branch has overrides syntax conversion patch
+    refspec: master-next
 
 machine: gem5-arm64