storage: Add infrastructure to manage XML namespace options

storage: Add infrastructure to manage XML namespace options

Introduce the virStoragePoolFSMountOptionsDef to be used to
manage the Storage Pool XML Namespace for mount options.

Using a new virStorageBackendNamespaceInit function, set the
virStoragePoolXMLNamespace into the _virStoragePoolOptions when
the storage backend is loaded.

Modify the storagepool.rng to allow for the usage of a different
XML namespace to parse the fs_mount_opts to be included with
the fs and netfs storage pool definitions.

Modify the storagepoolxml2xmltest to utilize a properly modified
XML file to parse and format the namespace for a netfs storage pool.

Signed-off-by: John Ferlan <jferlan@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>