migration: fix use-after-free of to_dst_file

hmp_savevm calls qemu_savevm_state(f), which sets to_dst_file=f in
global migration state. Then hmp_savevm closes f (g_free called).

Next access to to_dst_file in migration state (for example,
qmp_migrate_set_speed) will use it after it was freed.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20170225193155.447462-5-vsementsov@virtuozzo.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
