xenconfig: fix compilation error

Commit 03d0959a introduced a compilation error in
src/xenconfig/xen_xl.c on ARM. Found by Xen's osstest

http://logs.test-lab.xenproject.org/osstest/logs/116216/build-armhf-libvirt/6.ts-libvirt-build.log

src/xenconfig/xen_xl.c

@@ -1261,11 +1261,11 @@ xenFormatXLVnuma(virConfValuePtr list,
     numaVnode->list = NULL;
 
     /* pnode */
-    virBufferAsprintf(&buf, "pnode=%ld", node);
+    virBufferAsprintf(&buf, "pnode=%zu", node);
     xenFormatXLVnode(numaVnode, &buf);
 
     /* size */
-    virBufferAsprintf(&buf, "size=%ld", nodeSize);
+    virBufferAsprintf(&buf, "size=%zu", nodeSize);
     xenFormatXLVnode(numaVnode, &buf);
 
     /* vcpus */