pygrub: Add HybridISO support for PyGrub2

pygrub: 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.

Signed-off-by: Philipp Hahn <hahn@univention.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>