diff -urNp -x '*.orig' xen-4.13.1.org/tools/pygrub/src/pygrub xen-4.13.1/tools/pygrub/src/pygrub
--- xen-4.13.1.org/tools/pygrub/src/pygrub	2021-03-25 22:42:14.797689878 +0100
+++ xen-4.13.1/tools/pygrub/src/pygrub	2021-03-25 22:42:15.517685792 +0100
@@ -129,6 +129,7 @@ def get_partition_offsets(file):
     fd = os.open(file, os.O_RDONLY)
     buf = os.read(fd, 512)
     os.close(fd)
+    offzerocount = 0
     for poff in (446, 462, 478, 494): # partition offsets
 
         # MBR contains a 16 byte descriptor per partition
@@ -138,6 +139,7 @@ def get_partition_offsets(file):
         
         # offset == 0 implies this partition is not enabled
         if offset == 0:
+            offzerocount += 1
             continue
 
         if type == FDISK_PART_SOLARIS or type == FDISK_PART_SOLARIS_OLD:
@@ -163,6 +165,9 @@ def get_partition_offsets(file):
     # (e.g. grubs) may contain the same signature.
     if not part_offs: part_offs = [0]
 
+    if offzerocount == 4:
+        # Might be a grub boot sector pretending to be an MBR
+        part_offs.append(0)
     return part_offs
 
 class GrubLineEditor(curses.textpad.Textbox):