util: Introduce and use virObjectRWLockWrite
    
    Instead of making virObjectLock be the entry point for two
    different types of locks, let's create a virObjectRWLockWrite API
    which will only handle the virObjectRWLockableClass objects.
    
    Use the new virObjectRWLockWrite for the virdomainobjlist code
    in order to handle the Add, Remove, Rename, and Load operations
    that need to be very synchronous.
    
    Signed-off-by: John Ferlan <jferlan@redhat.com>