libxl: replace 'enabled' with 'unknown' in libxl_passthrough enumeration

libxl: replace 'enabled' with 'unknown' in libxl_passthrough enumeration

This is mostly a cosmetic patch to avoid the default enumeration value
being 'enabled'. The only non-cosmetic parts are in xl_parse.c where it now
becomes necessary to explicitly parse the 'enabled' value for xl.cfg
'passthrough' option, and error on the value 'unknown', because there is no
longer a direct mapping between valid xl.cfg values and the enumeration.

Suggested-by: Ian Jackson <ian.jackson@eu.citrix.com>
Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>