migration: use qemu_free() instead of free().

Although there is no difference, other migration related code use
qemu_free(), and it should be better to be consistent.

Signed-off-by: Yoshiaki Tamura <tamura.yoshiaki@lab.ntt.co.jp>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/migration.c b/migration.c
index fbf2339..64ed36e 100644
--- a/migration.c
+++ b/migration.c
@@ -396,7 +396,7 @@
         s->state = MIG_STATE_CANCELLED;
         migrate_fd_cleanup(s);
     }
-    free(s);
+    qemu_free(s);
 }
 
 void migrate_fd_wait_for_unfreeze(void *opaque)