vircgroup: register cgroup v2 backend
vircgroup: register cgroup v2 backend

All mandatory callbacks are implemented for cgroup v2 backend so we
can register it now.

Signed-off-by: Pavel Hrdina <d4772d05997b8abf035041e3b4f4996380ea7e7a@redhat.com>

jfehlig/libvirt,eskultety/libvirt,eskultety/libvirt,jfehlig/libvirt,zippy2/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,libvirt/libvirt,olafhering/libvirt,fabianfreyer/libvirt,crobinso/libvirt,zippy2/libvirt,jardasgit/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,crobinso/libvirt,zippy2/libvirt,libvirt/libvirt,nertpinx/libvirt,jardasgit/libvirt,olafhering/libvirt,eskultety/libvirt,libvirt/libvirt,olafhering/libvirt,nertpinx/libvirt,nertpinx/libvirt,andreabolognani/libvirt,crobinso/libvirt,libvirt/libvirt,andreabolognani/libvirt,crobinso/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,eskultety/libvirt,olafhering/libvirt,jardasgit/libvirt,nertpinx/libvirt,jfehlig/libvirt,zippy2/libvirt,eskultety/libvirt,jardasgit/libvirt,jfehlig/libvirt