The verbosity must be enabled at two levels: at compile time
via an enum that switches the availability of verbosity code
and via two members of 'Mmio' named '_read_verbose' and
'_write_verbose'. The latter are initialized to 0 (change this to
enable verbosity globally) and can be used to locally enable
(or disable) verbosity in deriving classes.
Ref #753
0c7c285079