$(XEN_ROOT) absolutification fixes for ioemu-remote (incl stubdom)

$(XEN_ROOT) absolutification fixes for ioemu-remote (incl stubdom)

 * Move code for generating an absolute version of XEN_ROOT
   into a common make variable set in Config.mk
 * Use this common code when invoking make -C ioemu-dir clean
   from tools/, which avoids a problem where `make clean' fails
   because qemu's (ioemu-remote's) build system wants to run
   `make clean' in `tests' but XEN_ROOT is a confection involving
   ../'s.
 * Use this common code in stubdom/Makefile, instead of $(abspath...)
   as the latter is a relatively new feature in GNU make and is not
   available in all the places that we want to be able to build
   (cf c/s 17997:3f23e01d31985899dbd1660b166f229f1ee74292)

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>