rwhandler: simplified way to register for mem/io

rwhandler: simplified way to register for mem/io

Some users prefer a single callback with length passed as parameter to
using b/w/l callbacks. It would maybe be cleaner to just pass length to
existing callbacks but that's a lot of churn.  So for now add a wrapper.
For convenience use pcibus_t for address so a single callback can be
used for pci io and pci memory.

I did have to resort to preprocessor to reduce code duplication.  It is
however slightly more straightforward, and better contained than what we
had with pci_host_template.h. Again, it would go away if we just passed
len to existing callbacks.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>