vm.install-debian: increase default RAM for Stretch

For Stretch, the installer needs more than 256M.

slapos/recipe/vm.py

@@ -297,7 +297,7 @@ class InstallDebianRecipe(BaseRecipe):
           for k, v in p.items())))
 
         vm = join(location, dist + '.img')
-        args = self.getQemuBasicArgs(dist, 256, unsafe=True)
+        args = self.getQemuBasicArgs(dist, 384, unsafe=True)
         open_flags = os.O_CREAT | os.O_EXCL | os.O_WRONLY
         fd = os.open(vm, open_flags, 0o666)
         try: