pygrub: cope with configurations with submenus

pygrub: cope with configurations with submenus

The grub2 configuration file in Fedora 16 can have one or more
menuentrys in a submenu, with configuration of the form
    submenu "Xen 4.1" {
    menuentry ... {
    ...
    }
    }
(this example occurs when the xen hypervisor is installed on the
guest)

Ignore the submenu line and the corresponding }

Signed-off-by: Michael Young <m.a.young@durham.ac.uk>
Acked-by: Ian Campbell <ian.campbell@citrix.com>