esx: Make storage pool lookup by name and UUID more robust

esx: Make storage pool lookup by name and UUID more robust

Don't rely on summary.url anymore, because its value is different
between an esx:// and vpx:// connection. Use host.mountInfo.path
instead.

Don't fallback to lookup by UUID (actually lookup by absolute path)
in esxVI_LookupDatastoreByName when lookup by name fails. Add a
seperate function for this: esxVI_LookupDatastoreByAbsolutePath