crtools: Make new_cg_root_add setup global root too

This is to make it convenient for service to setup the same thing.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
rpc: Add "cgroup root" fields to criu_opts

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
libcriu: Add criu_add_cg_root() call

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>