If you look understarage and see all of your datasets, if you select the vdev you used (iscsivol in your case) and select the wrench icon (edit) its in there.
FreeNAS Docs - zvol
If your just booting VMs and watching the graph, it will only show what the host needs to boot the VMs. If you have a windows VM download CrystalDiskMark and run a test at least 4GB. post teh screenshot.