Escape backward slashes in vbox share name

That fixes invalid share names in Windows guests, where backward slash character is
used as a path separator.

plugins/providers/virtualbox/synced_folder.rb

@@ -85,7 +85,7 @@ module VagrantPlugins
       end
 
       def os_friendly_id(id)
-        id.gsub(/[\/]/,'_').sub(/^_/, '')
+        id.gsub(/[\/\\]/,'_').sub(/^_/, '')
       end
 
       # share_folders sets up the shared folder definitions on the