grub2 should ignore xen hypervisor EFI files when building menus

For a while now (I think about Trusty), the Xen hypervisor package installs two files into /boot. One is the classic hypervisor code (xen*.gz), the other is an EFI executable. Currently we create menu entries for both but only the classic code can be run from grub2. The EFI version is intended to be started directly from the EFI BIOS.
So I would propose the following patch to ignore the EFI files when creating the menu entries. I decided on positively matching the EFI file and skipping over it over that and not narrowing the match because that might still allow a change of compression scheme without needing to change grub2 code again.
The change should be easy to SRU into older releases, too. Though, since the Xen modified default selects the non-EFI submenu, it might not be critical enough to do so.