Name	Description
cluster
datacenter
datastore
host
nasMount
resourcePool
scsiAdapter
scsiPath
scsiTarget
scsiVolume
storagePod
vm
vmFile

         <simpleType xmlns="http://www.w3.org/2001/XMLSchema" xmlns:sms="urn:sms" name="EntityReferenceEntityType">
            <restriction base="xsd:string">
               <enumeration value="datacenter"/>
               <enumeration value="resourcePool"/>
               <enumeration value="storagePod"/>
               <enumeration value="cluster"/>
               <enumeration value="vm"/>
               <enumeration value="datastore"/>
               <enumeration value="host"/>
               <enumeration value="vmFile"/>
               <enumeration value="scsiPath"/>
               <enumeration value="scsiTarget"/>
               <enumeration value="scsiVolume"/>
               <enumeration value="scsiAdapter"/>
               <enumeration value="nasMount"/>
            </restriction>
         </simpleType>

Enum - EntityReferenceEntityType(sms.EntityReference.EntityType)

Enum Description