migration: do not overwrite zero pages

on incoming migration do not memset pages to zero if they already read as zero.
this will allocate a new zero page and consume memory unnecessarily. even
if we madvise a MADV_DONTNEED later this will only deallocate the memory
asynchronously.
Signed-off-by: NPeter Lieven <pl@kamp.de>
Signed-off-by: NJuan Quintela <quintela@redhat.com>

arch_init.c

@@ -835,6 +835,7 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
             }
 
             ch = qemu_get_byte(f);
+            if (ch != 0 || !is_zero_page(host)) {
                 memset(host, ch, TARGET_PAGE_SIZE);
 #ifndef _WIN32
                 if (ch == 0 &&
@@ -843,6 +844,7 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
                     qemu_madvise(host, TARGET_PAGE_SIZE, QEMU_MADV_DONTNEED);
                 }
 #endif
+            }
         } else if (flags & RAM_SAVE_FLAG_PAGE) {
             void *host;