pygrub: Allow scrolling of the list of entries

pygrub: Allow scrolling of the list of entries

When user wants to change entry in grub2 menu in pygrub, there
may be crash of pygrub in case of editing item ('e' key).

Crash on editing is caused longer entry list in case of grub2. As entry
window is 10 lines high, it can hold only 8 entries (2 lines for border).
Adding line outside of windows high causes crash. Patch add handling
for longer lists and scrolling through them.

Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>