Packet: Enable functional reads of partial data to packet class

This patch fixes a long standing defficiency in the packet class where
it was unable to handle finding data that partially satisfied a request.

This splits out changes made to the packet class in the checkercpu patch as requested by Ali. 

Compiles. No functional changes made from CheckerCPU patch to this patch for packet class, and CheckerCPU fully exercised this code path during testing.