Adding Public Get and Set APIs for Network Metadata

Adding Public Get and Set APIs for Network Metadata

This patch introduces public Get and Set APIs for modifying <title>,
<description> and <metadata> elements of the Network object.

- Added enum virNetworkMetadataType to select one of the above
  elements to operate on.
- Added error code and messages for missing metadata.
- Added public API implementation.
- Added driver support.

Signed-off-by: K Shiva Kiran <shiva_kr@riseup.net>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>