Clarify: GET /host/diskpool mapping to z/VM

[johnarwe]

GET /host/diskpool talks about "disk pools", which are not a native z/VM concept. Libvirt has storage pools, "storage" means memory (to CP) or disk (to DirMaint, which usually calls it DASD), so I understand that a different term is needed. Since Feilong is z/VM-specific, consumers need to be clear about what the API maps to; my best would be a DirMaint DASD Group, but there might be another animal in z/VM's disk menagerie that I have not run across yet.

[jichenjc]

disk pool = dasd group

this is legacy issue , we can add some PR to update to doc to indicate this legacy wording to make people know they are identical