Adding function to calculate difference between two addresses.

Change-Id: If64d4b5f342f063dd0394c7bcc8a2389a63fe702
diff --git a/src/main.c b/src/main.c
index 1df1802..6263c32 100644
--- a/src/main.c
+++ b/src/main.c
@@ -88,7 +88,7 @@
 	}
 
 	memiter_init(&cpio, initrd,
-		     pa_addr(params.initrd_end) - pa_addr(params.initrd_begin));
+		     pa_difference(params.initrd_begin, params.initrd_end));
 
 	/* Load all VMs. */
 	if (!load_primary(&cpio, params.kernel_arg, &primary_initrd, &ppool)) {