[libvirt] ESX: Check if a datastore is accessible before accessing its URL property

An inaccessible datastore has no valid URL property. * src/esx/esx_vi.c: esxVI_LookupDatastoreByName(): check if datastore is accessible before accessing its URL property * src/esx/esx_vmx.c: update to changed datastore properties PS: This patch has to be applied after the 12er patch series. Matthias