migration: reduce the count of strlen call

'strlen' is called three times in 'save_page_header', it's
inefficient.

Signed-off-by: Liang Li <liang.z.li@intel.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

migration/ram.c

diff --git a/migration/ram.c b/migration/ram.c
index 1e58cd3..7f007e6 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -382,16 +382,16 @@
  */
 static size_t save_page_header(QEMUFile *f, RAMBlock *block, ram_addr_t offset)
 {
-    size_t size;
+    size_t size, len;
 
     qemu_put_be64(f, offset);
     size = 8;
 
     if (!(offset & RAM_SAVE_FLAG_CONTINUE)) {
-        qemu_put_byte(f, strlen(block->idstr));
-        qemu_put_buffer(f, (uint8_t *)block->idstr,
-                        strlen(block->idstr));
-        size += 1 + strlen(block->idstr);
+        len = strlen(block->idstr);
+        qemu_put_byte(f, len);
+        qemu_put_buffer(f, (uint8_t *)block->idstr, len);
+        size += 1 + len;
     }
     return size;
 }