Use cherrypy's default dispatcher for subcollection

Currently, we override the dispatchers of resource StoragePool
and IsoPool when the request targets to storagevolumes it holds.
But it's not necessary to use a customized dispatcher. It can be
done by add an attribute to associate with the subcollcetion. In
this case,  it's 'storagevolumes'.

Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>