dracut: add a flag file indicating scrub-pages option support

Indicate when the dracut "qubes-vm-simple" module supports (re-)enabling
xen_scrub_pages option. This means the kernel can be safely booted with
xen_scrub_pages=0.

QubesOS/qubes-issues#1963

dracut/simple/Makefile

@@ -2,3 +2,5 @@ install:
 	install -d $(DESTDIR)/usr/lib/dracut/modules.d/90qubes-vm-simple
 	install module-setup.sh init.sh \
 		$(DESTDIR)/usr/lib/dracut/modules.d/90qubes-vm-simple/
+	# flag indicating the module will (re-)enable scrub-pages option
+	touch $(DESTDIR)/usr/lib/dracut/modules.d/90qubes-vm-simple/xen-scrub-pages-supported