This fixes command line tools and functions which use
lxc_global_config_value o get lxcpath but don't strip the trailing /
leading to mismatching command path (as lxc_container_new does strip the
path).

As lxcpath is typically a const and so can't easily be changed by the
caller, add the trick directly into lxc_global_config_value (having to
juggle a bit in there too to avoid trying to alter a const).
Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>