Don't check for kernel updates on OpenVZ.

Case 103689: On OpenVZ and Virtuozzo, the kernel is not under the control of
the guest, so warning about the kernel being out-of-date is not helpful.
Simply note that the kernel updates are not supported and that the user
should keep the host's kernel up to date.
Merge pull request #8 from bk2204/kernel-openvz

Don't check for kernel updates on OpenVZ.