[Libguestfs] SELinux relabel API

[Colin Walters]

> Dan Walsh helpfully pointed out to us that we've been doing it wrong
> all along :-)  A much better way to relabel is to run:
> 
>   setfiles /etc/selinux/targeted/contexts/files/file_contexts DIR

Yes, this is what I'm doing with OSTree.  However in the general cross
labeling case it has a subtle issue with PCRE:
http://comments.gmane.org/gmane.comp.security.selinux/20214

There is of course always the potential issue for incompatible future
changes in the file_contexts format.

My current workaround is:
https://github.com/cgwalters/rpm-ostree/commit/0cb346b798aead0fd544e2c9ef45f1817ba19434