qemu-iotests: fix 203 migration completion race

qemu-iotests: fix 203 migration completion race

There is a race between the test's 'query-migrate' QMP command after the
QMP 'STOP' event and completing the migration:

The test case invokes 'query-migrate' upon receiving 'STOP'.  At this
point the migration thread may still be in the process of completing.
Therefore 'query-migrate' can return 'status': 'active' for a brief
window of time instead of 'status': 'completed'.  This results in
qemu-iotests 203 hanging.

Solve the race by enabling the 'events' migration capability, which
causes QEMU to emit migration-specific QMP events that do not suffer
from this race condition.  Wait for the QMP 'MIGRATION' event with
'status': 'completed'.

Reported-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20180305155926.25858-1-stefanha@redhat.com
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>