qemu: change first arg of qemuDomainAttachChrDeviceAssignAddr()
    
    from virDomainDefPtr to virDomainObjPtr so that the function has
    access to the other parts of the virDomainObjPtr. Take advantage of
    this by removing the "priv" arg and retrieving it from the
    virDomainObjPtr instead.
    
    No functional change.