Add *-xen to the list of grub-install-common platforms.

include/grub/util/install.h

@@ -87,6 +87,8 @@ enum grub_install_plat
     GRUB_INSTALL_PLATFORM_ARM_EFI,
     GRUB_INSTALL_PLATFORM_MIPSEL_QEMU_MIPS,
     GRUB_INSTALL_PLATFORM_MIPS_QEMU_MIPS,
+    GRUB_INSTALL_PLATFORM_I386_XEN,
+    GRUB_INSTALL_PLATFORM_X86_64_XEN,
     GRUB_INSTALL_PLATFORM_MAX
   };