migration: fix missing assignment for has_x_checkpoint_delay

We forgot to assign true to params->has_x_checkpoint_delay parameter
in qmp_query_migrate_parameters.

Without this, qmp command 'query-migrate-parameters' doesn't show the
default value for x-checkpoint-delay option.

This also fixes the fact that HMP was relying on unspecified behavior by
reading x_checkpoint_delay without checking has_x_checkpoint_delay.

Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
