providers/virtualbox: package --base should use vboxsf

plugins/providers/virtualbox/synced_folder.rb

@@ -5,8 +5,11 @@ module VagrantPlugins
     class SyncedFolder < Vagrant.plugin("2", :synced_folder)
       def usable?(machine, raise_errors=false)
         # These synced folders only work if the provider if VirtualBox
-        machine.provider_name == :virtualbox &&
-          machine.provider_config &&
+        return false if machine.provider_name != :virtualbox
+
+        # This only happens with `vagrant package --base`. Sigh.
+        return true if !machine.provider_config
+
         machine.provider_config.functional_vboxsf
       end