Virtual Files Mime Type: use "vnd." prefix instead of "x-"

As reported in https://github.com/owncloud/client/issues/6717#issuecomment-413703567

mirall.desktop.in

@@ -8,7 +8,7 @@ GenericName=Folder Sync
 Icon=@APPLICATION_ICON_NAME@
 Keywords=@APPLICATION_NAME@;syncing;file;sharing;
 X-GNOME-Autostart-Delay=3
-MimeType=application/x-@APPLICATION_EXECUTABLE@;
+MimeType=application/vnd.@APPLICATION_EXECUTABLE@;
 
 # Translations
 Comment[oc]=@APPLICATION_NAME@ sincronizacion del client

src/gui/owncloud.xml.in

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <mime-info xmlns="http://www.freedesktop.org/standards/shared-mime-info">
-  <mime-type type="application/x-@APPLICATION_EXECUTABLE@">
+  <mime-type type="application/vnd.@APPLICATION_EXECUTABLE@">
     <comment>@APPLICATION_NAME@ virtual files</comment>
     <glob pattern="*.@APPLICATION_VIRTUALFILE_SUFFIX@"/>
   </mime-type>