fix segfault in setting migration speed

Hi,

Whoever wrote this migrate_set_speed function is totally stupid.

Any failed or completed migration keeps its state to allow probing of
migration data, but has no associated file anymore. It is, thus,
possible to crash qemu by calling migrate_set_speed after a migration
is finished (or failed, or cancelled), but before another one starts.

This patch fixes it.

Signed-off-by: Glauber Costa <glommer@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
