block/vdi: Fix wrong size in conditionally used memset, memcmp

Error report from cppcheck:
block/vdi.c:122: error: Using sizeof for array given as function argument returns the size of pointer.
block/vdi.c:128: error: Using sizeof for array given as function argument returns the size of pointer.

Fix both by setting the correct size.

The buggy code is only used when QEMU is build without uuid support.
The bug is not critical, so there is no urgent need to apply it to
old versions of QEMU.

Cc: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: NStefan Weil <weil@mail.berlios.de>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/vdi.c

@@ -119,13 +119,13 @@ void uuid_unparse(const uuid_t uu, char *out);
 #if !defined(CONFIG_UUID)
 void uuid_generate(uuid_t out)
 {
-    memset(out, 0, sizeof(out));
+    memset(out, 0, sizeof(uuid_t));
 }
 
 int uuid_is_null(const uuid_t uu)
 {
     uuid_t null_uuid = { 0 };
-    return memcmp(uu, null_uuid, sizeof(uu)) == 0;
+    return memcmp(uu, null_uuid, sizeof(uuid_t)) == 0;
 }
 
 void uuid_unparse(const uuid_t uu, char *out)