Unfortunately, there is not method in SoftLayer_VIrtual_Guest service which can help to retrieve portable storage volumes information.
As workaround, It's possible to get this information through SoftLayer_Account::getPortableStorageVolumes but we need to use an Object Filter, below a request which can help to retrieve Portable Storage for a specific Virtual Guest:
curl -k "https://$user:$apiKey@api.softlayer.com/rest/v3.1/SoftLayer_Account/getPortableStorageVolumes?objectFilter={"portableStorageVolumes":{"blockDevices":{"guest":{"id":{"operation":123123123}}}}}"
Replace: $user, $apiKey and 123123123(VSI id) with your own information
If you have a problem with encoding, try this:
curl -k "https://$user:$apiKey@api.softlayer.com/rest/v3.1/SoftLayer_Account/getPortableStorageVolumes?objectFilter=%7B%22portableStorageVolumes%22%3A%7B%22blockDevices%22%3A%7B%22guest%22%3A%7B%22id%22%3A%7B%22operation%22%3A123123123%7D%7D%7D%7D%7D"
Replace: $user, $apiKey and 123123123(VirtualGuest's id)