Bug #23812: Add HybridISO support for PyGrub2

grub-mkrescue internally uses xorriso, which generates a so-called "Hybrid
ISO": The ISO images also contains a DOS partition table, which allows the
identical ISO file to be stored on an USB stick for booting from it. This
breaks PyGrub, since it (wrongly) detects only the DOS partition table and uses
the first partition instead of the complete ISO file.

Add a check to detect HybridISO files and use offset 0 in addition to partition
table parsing.
--- a/tools/pygrub/src/pygrub
+++ b/tools/pygrub/src/pygrub
@@ -40,15 +40,20 @@ def enable_cursor(ison):
     except _curses.error:
         pass
 
-def is_disk_image(file):
+DISK_TYPE_RAW, DISK_TYPE_HYBRIDISO, DISK_TYPE_DOS = range(3)
+def identify_disk_image(file):
+    """Detect DOS partition table or HybridISO format."""
     fd = os.open(file, os.O_RDONLY)
-    buf = os.read(fd, 512)
+    buf = os.read(fd, 0x8006)
     os.close(fd)
 
     if len(buf) >= 512 and \
            struct.unpack("H", buf[0x1fe: 0x200]) == (0xaa55,):
-        return True
-    return False
+        # HybridISO contains a DOS partition table for booting from USB devices, but really is an ISO image
+        if len(buf) >= 0x8006 and buf[0x8001:0x8006] == 'CD001':
+            return DISK_TYPE_HYBRIDISO
+        return DISK_TYPE_DOS
+    return DISK_TYPE_RAW
 
 SECTOR_SIZE=512
 DK_LABEL_LOC=1
@@ -87,12 +92,19 @@ FDISK_PART_SOLARIS_OLD=0x82
 FDISK_PART_GPT=0xee
 
 def get_partition_offsets(file):
-    if not is_disk_image(file):
+    image_type = identify_disk_image(file)
+    if image_type == DISK_TYPE_RAW:
         # No MBR: assume whole disk filesystem, which is like a 
         # single partition starting at 0
         return [0]
-
-    part_offs = []
+    elif image_type == DISK_TYPE_HYBRIDISO:
+        # A HybridISO contains an ISO filesystem at 0 in addition
+        # to the DOS partition table
+        part_offs = [0]
+    elif image_type == DISK_TYPE_DOS:
+        part_offs = []
+    else:
+        raise ValueError('Unhandled image type returnd by identify_disk_image(): %d' % (image_type,))
 
     fd = os.open(file, os.O_RDONLY)
     buf = os.read(fd, 512)