pygrub: fix non-interactive parsing of grub1 config files

pygrub: fix non-interactive parsing of grub1 config files

Changes to handle non-numeric default attributes for grub2 caused run_grub()
to attempt to index into the images list using a string. Pull out the code
that handles submenus into a new function and use that to ensure sel is
numeric.

Reported-by: David Scott <dave.scott@citrix.com>
Signed-off-by: Simon Rowe <simon.rowe@eu.citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-and-tested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>