virStorageSource: Eliminate 'volume' field

virStorageSource: Eliminate 'volume' field

While historically we've stored the 'pool' and 'image' properties of RBD
and gluster images in separate fields but they are presented in a single
field in the XML. This creates multiple points where they need to be
separated and combined.

Introduce helper 'virStorageSourceNetworkProtocolPathSplit' which will
do that at the point of use rather than everywhere in the code.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>