build: Fix distclean when repo location changes

If the path to xen-unstable.hg changes (i.e. you move the repo), the symlinks
inside xen-unstable.hg/stubdom/libxc-x86_[32|64]/ all become broken, which
breaks distclean because make attempts to clean inside those first and fails to
find Makefile (which is also a symlink).

Signed-off-by: Andrei Lifchits <andrei.lifchits@citrix.com>

stubdom/Makefile

@@ -501,7 +501,7 @@ clean:
 	$(MAKE) -C vtpmmgr clean
 	rm -fr grub-$(XEN_TARGET_ARCH)
 	rm -f $(STUBDOMPATH)
-	[ ! -d libxc-$(XEN_TARGET_ARCH) ] || $(MAKE) DESTDIR= -C libxc-$(XEN_TARGET_ARCH) clean
+	[ ! -e libxc-$(XEN_TARGET_ARCH)/Makefile ] || $(MAKE) DESTDIR= -C libxc-$(XEN_TARGET_ARCH) clean
 	-[ ! -d ioemu ] || $(MAKE) DESTDIR= -C ioemu clean
 	-[ ! -d xenstore ] || $(MAKE) DESTDIR= -C xenstore clean