iotests.py: Fix type check errors in wait_migration()

iotests.py: Fix type check errors in wait_migration()

Commit 1847a4a8c20 clarified that event_wait() can return None (though
only with timeout=0) and commit f12a282ff47 annotated it as returning
Optional[QMPMessage].

Type checks in wait_migration() fail because of the unexpected optional
return type:

iotests.py:750: error: Value of type variable "Msg" of "log" cannot be "Optional[Dict[str, Any]]"
iotests.py:751: error: Value of type "Optional[Dict[str, Any]]" is not indexable
iotests.py:754: error: Value of type "Optional[Dict[str, Any]]" is not indexable

Fortunately, the non-zero default timeout is used in the event_wait()
call, so we can make mypy happy by just asserting this.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20201027163806.290960-2-kwolf@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>