pygrub detection: Prefer /usr/lib/xen-default over /usr/lib/xen-x.y

Otherwise a versioned pygrub path will be used (e.g.
/usr/lib/xen-4.1/bin/pygrub  in the case of Debian Wheezy) and then
all host configurations would need to be updated when dist-upgrading
to Debian Jessie with Xen 4.3.

In case you already ran into this issue, the following command should
fix the issue for you:

fgrep -l xen-4.1/bin/pygrub -r /etc/xen/ | xargs sed -e 's:xen-4.1/bin/pygrub:xen-default/bin/pygrub:g' -i