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-unix.c b/migration-unix.c
index 64bfa31..94b7022 100644
--- a/migration-unix.c
+++ b/migration-unix.c
@@ -35,11 +35,11 @@
 
     if (fd < 0) {
         DPRINTF("migrate connect error\n");
-        s->migration_file = NULL;
+        s->file = NULL;
         migrate_fd_error(s);
     } else {
         DPRINTF("migrate connect success\n");
-        s->migration_file = qemu_fopen_socket(fd, "wb");
+        s->file = qemu_fopen_socket(fd, "wb");
         migrate_fd_connect(s);
     }
 }