anaconda: do not have any requirements on /boot in EFI mode

EFI boot uses only /boot/efi, so /boot may be even on encrypted volume,
LVM, btrfs or anything else. Instead of allowing just LVM, override the
whole check for /boot.

Fixes QubesOS/qubes-issues#1721

anaconda/pyanaconda/bootloader.py

@@ -1865,6 +1865,10 @@ class XenEFI(EFIGRUB):
     def write_config_post(self):
         pass
 
+    def is_valid_stage2_device(self, device, linux=True, non_linux=False):
+        """ XenEFI doesn't use stage2 at all, so allow anything here """
+        return True
+
     write_config = BootLoader.write_config
 
 class MacEFIGRUB(EFIGRUB):