tools/pygrub: Fix default when out of range

tools/pygrub: Fix default when out of range

This is the patch to fix pyGrub default value when it's being set out of
range. This patch makes the quiet and interactive mode select the same
default image when the default value for boot entry is out of range,
i.e. when the guest is having wrong configuration in it's boot loader
(like 3 entries with default mistakenly set to 10 etc).

When the boot entry number is being set out of range it falls back to 0
(first entry of boot loader).

Signed-off-by: Michal Novotny <minovotn@redhat.com>