lxc: allow use of lxc:///system URI as preferred format

lxc: allow use of lxc:///system URI as preferred format

Historically we have used a bare lxc:/// URI for connecting to LXC. This
is different from our practice with QEMU, UML, Parallels, Libxl, BHyve
and VirtualBox drivers, which all use a path of '/system' or '/session'
or both.

By making LXC allow '/system', we have fully standardized on the use of
either '/system' or '/session' for all the stateful drivers that run
inside libvirtd.

Support for lxc:/// is of course maintained for back-compat.

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