makefile/windows: install qvm-tools with .py extension

So system will automatically run them with python interpreter (wherever
it is installed). This require to have ".py" in PATHEXT variable.

qvm-tools/Makefile

@@ -1,10 +1,14 @@
 BINDIR ?= /usr/bin
+ifeq ($(OS),Windows_NT)
+PYTHONEXEEXT = .py
+endif
 
 all:
 	true
 
 install:
 	mkdir -p $(DESTDIR)$(BINDIR)
-	cp qvm-* $(DESTDIR)$(BINDIR)
-	cp qubes-* $(DESTDIR)$(BINDIR)
+	for f in qvm-* qubes-*; do \
+		cp $$f $(DESTDIR)$(BINDIR)/$$f$(PYTHONEXEEXT); \
+	done