vm: always use GPT

It's a nightmare to convert from DOS to GPT (required when moving to a
disk bigger than 2TB), because GRUB can only be installed reliably
from the system to boot.

slapos/recipe/vm.py

@@ -164,7 +164,8 @@ class InstallDebianRecipe(BaseRecipe):
     passwd/make-user = false
     passwd/root-login = true
     partman-auto/method = regular
-    partman-auto/expert_recipe = : 1 1 -1 - $primary{ } $bootable{ } method{ format } format{ } use_filesystem{ } filesystem{ xfs } mountpoint{ / } options/noatime{ } .
+    partman-auto/expert_recipe = : 1 1 1 - method{ biosgrub } . 1 2 -1 - method{ format } format{ } use_filesystem{ } filesystem{ xfs } mountpoint{ / } options/noatime{ } .
+    partman-partitioning/default_label = gpt
     """
 
   # XXX: The mapping should be automatically computed from the