tools: pass --libdir to qemu configure

On an x86_64 environment, when we configure xen using:

  ./configure --prefix=/usr --libdir=/usr/lib64
  
only --prefix=/usr is passed to qemu configure. Then qemu will use
$(PREFIX)/lib as libdir, which causes inconsistent with other xen tools.

Signed-off-by: Zhigang Wang <zhigang.x.wang@xxxxxxxxxx>
diff --git a/tools/Makefile b/tools/Makefile
index 3675515..93c79e7 100644
--- a/tools/Makefile
+++ b/tools/Makefile
@@ -188,6 +188,7 @@ subdir-all-qemu-xen-dir: qemu-xen-dir-find
 	$$source/configure --enable-xen --target-list=i386-softmmu \
 		$(QEMU_XEN_ENABLE_DEBUG) \
 		--prefix=$(PREFIX) \
+		--libdir=$(LIBDIR) \
 		--source-path=$$source \
 		--extra-cflags="-I$(XEN_ROOT)/tools/include \
 		-I$(XEN_ROOT)/tools/libxc \