qapi/migration.json: fix the description for "query-migrate" output

qapi/migration.json: fix the description for "query-migrate" output

In the return for command "query-migrate", time information like
"total-time", "setup-time", "downtime", is not included in ram
json-object.

So fix the description in migration.json by unpacking those information
from ram json-object.

Signed-off-by: jialina01 <jialina01@baidu.com>
Signed-off-by: chaiwen <chaiwen@baidu.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>