migration: Reset rather than destroy main_thread_load_event

migration_incoming_state_destroy doesn't really destroy, it cleans up.
After a loadvm it's called, but the loadvm command can be run twice,
and so destroying an init-once mutex breaks on the second loadvm.

Reported-by: Stafford Horne <shorne@gmail.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20170825141940.20740-2-dgilbert@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Tested-by: Stafford Horne <shorne@gmail.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>