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

@@ -835,14 +835,16 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
             }
 
             ch = qemu_get_byte(f);
-            memset(host, ch, TARGET_PAGE_SIZE);
+            if (ch != 0 || !is_zero_page(host)) {
+                memset(host, ch, TARGET_PAGE_SIZE);
 #ifndef _WIN32
-            if (ch == 0 &&
-                (!kvm_enabled() || kvm_has_sync_mmu()) &&
-                getpagesize() <= TARGET_PAGE_SIZE) {
-                qemu_madvise(host, TARGET_PAGE_SIZE, QEMU_MADV_DONTNEED);
-            }
+                if (ch == 0 &&
+                    (!kvm_enabled() || kvm_has_sync_mmu()) &&
+                    getpagesize() <= TARGET_PAGE_SIZE) {
+                    qemu_madvise(host, TARGET_PAGE_SIZE, QEMU_MADV_DONTNEED);
+                }
 #endif
+            }
         } else if (flags & RAM_SAVE_FLAG_PAGE) {
             void *host;