api: Add osv::version for querying OSv version

The uname() function returns a fake Linux version number for application
compatibility.  Add a new osv::version() API that returns OSv version
that can be used by the management code.

Signed-off-by: Amnon Heiman <amnon@cloudius-systems.com>
[ penberg: cleanups ]
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>