vm: switch to XFS

ext4 was chosen for the 'discard' option, but formatting takes too
much time and space. I was surprised to see my VM being paused
(no left space on host) because of ext4lazyinit.

slapos/recipe/vm.py

@@ -164,7 +164,7 @@ class InstallDebianRecipe(BaseRecipe):
     passwd/make-user = false
     passwd/root-login = true
     partman-auto/method = regular
-    partman-auto/expert_recipe = : 1 1 -1 ext4 $primary{ } $bootable{ } method{ format } format{ } use_filesystem{ } filesystem{ ext4 } mountpoint{ / } options/discard{ } options/noatime{ } .
+    partman-auto/expert_recipe = : 1 1 -1 - $primary{ } $bootable{ } method{ format } format{ } use_filesystem{ } filesystem{ xfs } mountpoint{ / } options/noatime{ } .
     """
 
   # XXX: The mapping should be automatically computed from the