kernel-install: adjust EFI check to look for xen.cfg

Even if EFI directory is present it may not be populated. kernel-install
part care specifically about xen.cfg file, so check it explicitly. If
grub2-efi is in use, the file wont be there and the script isn't
supposed to do anything.

system-config/kernel-xen-efi.install

@@ -24,7 +24,7 @@ else
     EFI_DIR="$ESP_MOUNTPOINT$EFI_DIR"
 fi
 
-if [ ! -d "$EFI_DIR" ]; then
+if [ ! -r "$EFI_DIR/xen.cfg" ]; then
     # non-EFI system
     exit 0;
 fi