virt-host-validate: check for required cgroups
    
    Extend the virt-host-validate checks to see if the required
    cgroups are compiled into the kernel and that they are
    mounted on the system. The cgroups are all optional except
    for 3 that LXC mandates
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>