test-qmp-event: Simplify and tighten event_test_emit()

Use qdict_get_qdict() and qdict_get_try_int() to simplify.

While there, add a sanity check for seconds.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1487363905-9480-7-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
