Store and retrieve vlan description to/from shared memory.

The userspace tools (swcli, swctl etc.) do not make use of these changes. For
a demo of how to use the set_vlan_desc and get_vlan_desc, set the description of
a vlan as usual and call "show vlan" from swcli.

TODO: modify current implementation to use shared memory instead of syscall for
vlan description, see issue 5 for some starting points.