domain: conf: Better errors on bad os <type> values

domain: conf: Better errors on bad os <type> values

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.