hw/i386/Makefile.obj: use $(PYTHON) to run .py scripts consistently

hw/i386/Makefile.obj: use $(PYTHON) to run .py scripts consistently

$(PYTHON) is a Make variable which is set by configure.
In all other places over the tree, .py files are run from
Makefiles using this variable, except of a single leftover
in hw/i386/Makefile.obj (and a nearby place in there uses
$(PYTHON) correctly).  Fix this leftover too.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Cc: Luigi Rizzo <rizzo@iet.unipi.it>
Reviewed-by:: Stefan Weil <sw@weilnetz.de>