Fix diskimg OpenImage

The OpenImage method had an overload that took void*.  This turns out
to be a bad idea, because void* matches any pointer type that didn't
match something else.  So the WCHAR* filenames were going to the "open
from buffer" method rather than the "open from file" variant.

A less important issue is whether open-from-buffer should take a const
or non-const pointer.  If the "readOnly" boolean flag is not set, then
the contents can be altered and const is inappropriate.  The best course
seems to be to drop the boolean flag as an argument, and just have two
different methods.