esx: set vmfs fs type for vmfs-based datastores

This way they are correctly represented:
  <source>
    <format type='vmfs'/>
  </source>
... instead of 'auto'.
Signed-off-by: NPino Toscano <ptoscano@redhat.com>
Reviewed-by: NCole Robinson <crobinso@redhat.com>

src/esx/esx_storage_backend_vmfs.c

@@ -531,6 +531,7 @@ esxStoragePoolGetXMLDesc(virStoragePoolPtr pool, unsigned int flags)
         }
     } else if (esxVI_VmfsDatastoreInfo_DynamicCast(info)) {
         def.type = VIR_STORAGE_POOL_FS;
+        def.source.format = VIR_STORAGE_POOL_FS_VMFS;
         /*
          * FIXME: I'm not sure how to represent the source and target of a
          * VMFS based datastore in libvirt terms