Pull definition of structs out of vircgroup.c to vircgrouppriv.h

Pull definition of structs out of vircgroup.c to vircgrouppriv.h

The definition of structs for cgroups are kept in vircgroup.c since
they are intended to be private from users of the API. To enable
effective testing, however, they need to be accessible. To address
the latter issue, without compronmising the former, this introduces
a new vircgrouppriv.h file to hold the struct definitions.

To prevent other files including this private header, it requires
that __VIR_CGROUP_ALLOW_INCLUDE_PRIV_H__ be defined before inclusion

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