Add support for LXC specific SELinux configuration

Add support for LXC specific SELinux configuration

The SELinux policy for LXC uses a different configuration file
than the traditional svirt one. Thus we need to load
/etc/selinux/targeted/contexts/lxc_contexts which contains
something like this:

 process = "system_u:system_r:svirt_lxc_net_t:s0"
 file = "system_u:object_r:svirt_lxc_file_t:s0"
 content = "system_u:object_r:virt_var_lib_t:s0"

cleverly designed to be parsable by virConfPtr

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>