If no <os><type> was specified:
before: unknown OS type no OS type
after : xml error: an os <type> must be specified
If an <os><type> is specified that's not in our capabiliities data:
before: unknown OS type: $type
after : unsupported configuration: no support found for os <type> '$type'
VIR_ERR_OS_TYPE is now unused (as it should be frankly) so drop its strings
as well to save our translators some effort.