libipt, insn: add support for a read memory callback

Add an instruction decoder function for providing a read memory callback
function to be used when the memory cannot be found in the image.

This allows users to handle memory images themselves.

Change-Id: I4a715d684c472061a6a2f9a6ad28c2624a33f3ea
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>