Pass argv to the init binary of LXC, using a new <initarg> element.

* docs/formatdomain.html.in: Document <os> usage for containers
* docs/schemas/domaincommon.rng: Add <initarg> element
* src/conf/domain_conf.c, src/conf/domain_conf.h: parsing and
  formatting of <initarg>
* src/lxc/lxc_container.c: Setup LXC argv
* tests/Makefile.am, tests/lxcxml2xmldata/lxc-systemd.xml,
  tests/lxcxml2xmltest.c, tests/testutilslxc.c,
  tests/testutilslxc.h: Test parsing/formatting of LXC related
  XML parts