Get rid of hardcoded pathes for stubdom and hotplug scripts

Get rid of hardcoded pathes for stubdom and hotplug scripts

- Have the buid system generate a file which exports the install paths
   for the hotplug scripts and stubdom / stubdom-dm
- Move file generation code from tools/python/Makefile into a gmake
   macro
  in Config.mk to avoid maintenance of three duplicates each with its
   own
  tweaks and bugs
- Export gmake variables into ioemu as shell variables
  for upcoming ioemu patches
- Do above as a gmake macro to avoid maintenance of several duplicates
- Adjust hotplug scripts to find the right xen binaries from the
  install directory
- Adjust stubdom-dm to use the install directories

Signed-off-by: Christoph Egger <Christoph.Egger@amd.com>