Add support for sVirt in the LXC driver

Add support for sVirt in the LXC driver

For the sake of backwards compat, LXC guests are *not*
confined by default. This is because it is not practical
to dynamically relabel containers using large filesystem
trees. Applications can create confined containers though,
by giving suitable XML configs

* src/Makefile.am: Link libvirt_lxc to security drivers
* src/lxc/libvirtd_lxc.aug, src/lxc/lxc_conf.h,
  src/lxc/lxc_conf.c, src/lxc/lxc.conf,
  src/lxc/test_libvirtd_lxc.aug: Config file handling for
  security driver
* src/lxc/lxc_driver.c: Wire up security driver functions
* src/lxc/lxc_controller.c: Add a '--security' flag to
  specify which security driver to activate
* src/lxc/lxc_container.c, src/lxc/lxc_container.h: Set
  the process label just before exec'ing init.