It's possible to get that information using Object Filters, you can try the following request:
https://[username]:[aoiKey]@api.softlayer.com/rest/v3.1/SoftLayer_Network_Storage/[storageId]/getAllowedVirtualGuests?objectFilter={"allowedVirtualGuests":{"id":{"operation": "9222721"}}}&objectMask=mask[fullyQualifiedDomainName,allowedHost[name,credential[username,password], assignedVolumes[id, username]]]
Method: Get
Replace: [username], [apiKey], [storageId] and 9222721 (The VM's identifier) with your own information.
Just in case, it's not possible to apply filter over fullyQualifiedDomainName, but it's possible to use the hostname, so, in case that you want to get information through this property, try this:
https://[username]:[apiKey]@api.softlayer.com/rest/v3.1/SoftLayer_Network_Storage/[storageId]/getAllowedVirtualGuests?objectFilter={"allowedVirtualGuests":{"hostname":{"operation": "testhost"}}}&objectMask=mask[fullyQualifiedDomainName,allowedHost[name,credential[username,password], assignedVolumes[id, username]]]
Method: Get
Replace: [username], [apiKey], [storageId] and testhost (The VM's hostname) with your own information.
If you want to get this information for baremetal servers, you need to use the following method:
References: