pygrub: Fix "a" entry editing in grub2

pygrub: Fix "a" entry editing in grub2

When user wants to change entry in grub2 menu in pygrub, there's no
response in case of appending command line arguments ('a' key).

Append malfunction is caused by change of keyword used for kernel
record. Grub uses 'kernel' for line with linux kernel but grub2 uses
'linux' instead. This patch adds checking for both grub 1 and 2 keywords.

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