Use $localstatedir/log/lxc for default log path

Use $localstatedir/log/lxc for default log path

When we install lxc by manual (configure; make; make install),
all files are installed under /usr/local/. Configuration files
and setting files of containers are stored under /usr/local/ too,
however, only log files are stored under /var/log/ not
/usr/local/var/log.

This patch changes the default log path to $localstatedir/log/lxc
(by default $localstatedir is /usr/local/var) where is an ordinary
directory, which is probably expected and unsurprising.

Signed-off-by: Ryota Ozaki <ozaki.ryota@gmail.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>