Logging: don't confuse command line and config file specified values

Currently if loglevel/logfile are specified on command line in a
program using LXC api, and that program does any
container->save_config(), then the new config will be saved with the
loglevel/logfile specified on command line.  This is wrong, especially
in the case of

cat > lxc.conf << EOF
lxc.logfile=a
EOF

lxc-create -t cirros -n c1 -o b

which will result in a container config with lxc.logfile=b.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>