providers/virtualbox: make ID os-friendly for Windows

plugins/providers/virtualbox/synced_folder.rb

@@ -14,7 +14,7 @@ module VagrantPlugins
           hostpath = Vagrant::Util::Platform.cygwin_windows_path(data[:hostpath])
 
           defs << {
-            name: id,
+            name: os_friendly_id(id),
             hostpath: hostpath.to_s,
             transient: data[:transient],
           }
@@ -53,7 +53,8 @@ module VagrantPlugins
 
             # Mount the actual folder
             machine.guest.capability(
-              :mount_virtualbox_shared_folder, id, data[:guestpath], data)
+              :mount_virtualbox_shared_folder,
+              os_friendly_id(id), data[:guestpath], data)
           else
             # If no guest path is specified, then automounting is disabled
             machine.ui.detail(I18n.t("vagrant.actions.vm.share_folders.nomount_entry",
@@ -72,6 +73,10 @@ module VagrantPlugins
       def driver(machine)
         machine.provider.driver
       end
+
+      def os_friendly_id(id)
+        id.gsub(/[\/]/,'_').sub(/^_/, '')
+      end
     end
   end
 end