migration: Never call twice qemu_target_page_size()

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
diff --git a/migration/savevm.c b/migration/savevm.c
index d59e976..0bef031 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -1685,6 +1685,7 @@
 {
     PostcopyState ps = postcopy_state_set(POSTCOPY_INCOMING_ADVISE);
     uint64_t remote_pagesize_summary, local_pagesize_summary, remote_tps;
+    size_t page_size = qemu_target_page_size();
     Error *local_err = NULL;
 
     trace_loadvm_postcopy_handle_advise();
@@ -1741,13 +1742,13 @@
     }
 
     remote_tps = qemu_get_be64(mis->from_src_file);
-    if (remote_tps != qemu_target_page_size()) {
+    if (remote_tps != page_size) {
         /*
          * Again, some differences could be dealt with, but for now keep it
          * simple.
          */
         error_report("Postcopy needs matching target page sizes (s=%d d=%zd)",
-                     (int)remote_tps, qemu_target_page_size());
+                     (int)remote_tps, page_size);
         return -1;
     }