migration: Stop postcopy fault thread before notifying

migration: Stop postcopy fault thread before notifying

POSTCOPY_NOTIFY_INBOUND_END handlers will remove userfault fds
from the postcopy_remote_fds array which could be still in
use by the fault thread. Let's stop the thread before
notification to avoid possible accessing wrong memory.

Fixes: 46343570c06e ("vhost+postcopy: Wire up POSTCOPY_END notify")
Cc: [email protected]
Signed-off-by: Ilya Maximets <[email protected]>
Message-Id: <20181008160536[email protected]>
Reviewed-by: Dr. David Alan Gilbert <[email protected]>
Reviewed-by: Maxime Coquelin <[email protected]>
Signed-off-by: Dr. David Alan Gilbert <[email protected]>