qemu: Implement virDomain{Get,Set}Time
    
    One caveat though, qemu-ga is expecting time and returning time
    in nanoseconds. With all the buffering and propagation delay, the
    time is already wrong once it gets to the qemu-ga, but there's
    nothing we can do about it.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>