Debugger: Clean up DwarfFile::_GetDebugInfoPath().

- Fix a regression introduced when refactoring for the removal of /boot/common,
and adjust to use BPathFinder::FindPath() with implicit existence check instead
of manual iteration.
Add KDL command "mapping"

* VMTranslationMap:
  - Add DebugPrintMappingInfo(): Given a virtual address it is supposed
    to print the paging structure information for that address. To be
    implemented by derived classes.
  - Add DebugGetReverseMappingInfo(): Given a physical addresss it is
    supposed to find all virtual addresses mapped to it. To be
    implemented by derived classes.
* X86VMTranslationMapPAE: Implement the new methods
  DebugPrintMappingInfo() and DebugGetReverseMappingInfo().
* Add KDL command "mapping". It supports both virtual address lookups
  and reverse lookups.