introduce lxcapi_rename for renaming containers

lxcapi_rename implemented as a convenience function as lately
I find myself in a need to rename a container due to a
typo in its name. I could have started over but didn't want
to spend more time (to installing extra packages and changing
their configuration) on it.

c->clone() followed by c->destroy() did the trick for me and I
though it could be helpful to the other people, so here it is.

Signed-off-by: S.Çağlar Onur <caglar@10ur.org>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>