init.d: Fix xencommons init script chkconfig configuration

init.d: Fix xencommons init script chkconfig configuration

Fix the xencommons init script chkconfig configuration since the
priority was missing here and chkconfig was complaining about invalid
chkconfig script so I added both start and stop priorities to the
chkconfig line and it was working fine, the script was successfully
added to chkconfig when using `chkconfig --add xencommons` and
management using the chkconfig utility was now possible.

Tested on RHEL-5 Server with Xen-4.1-unstable installed, running on
PVops kernel 2.6.32.15 and it was working fine.

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