qemu_cgroup: Remove unnecessary virQEMUDriverPtr arguments
    
    Since commit 2e6ecba1bcac, the pointer to the qemu driver is saved in
    domain object's private data and hence does not have to be passed as
    yet another parameter if domain object is already one of them.
    
    This is a first (example) patch of this kind of clean up, others will
    hopefully follow.
    
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>