Fix Debian-7.0-b4-amd64-netboot os_type_id

The Debian Wheezy (7.0-b4) os_type_id for amd64-netboot is incorrectly
set to 'Debian'. It should be 'Debian_64' to allow boot.
Merge pull request #509 from botimer/wheezy-amd64

Fix Debian-7.0-b4-amd64-netboot os_type_id