migration: eliminate s->migration_file

The indirection is useless now.  Backends can open s->file directly.

Reviewed-by: Orit Wasserman <owasserm@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
diff --git a/migration-fd.c b/migration-fd.c
index 07c758a..3d4613c 100644
--- a/migration-fd.c
+++ b/migration-fd.c
@@ -36,7 +36,7 @@
     if (fd == -1) {
         return;
     }
-    s->migration_file = qemu_fdopen(fd, "wb");
+    s->file = qemu_fdopen(fd, "wb");
 
     migrate_fd_connect(s);
 }