libxl: introduce LIBXL_DOMAIN_TYPE_INVALID

libxl: introduce LIBXL_DOMAIN_TYPE_INVALID

To avoid recent gcc complaining about:
libxl.c: In function 'libxl_primary_console_exec':
libxl.c:1233:9: error: case value '4294967295' not in enumerated type 'libxl_domain_type' [-Werror=switch]

Also:
 - have all the call sites of libxl__domain_type() return with error in
   case the function returns LIBXL_DOMAIN_TYPE_INVALID;
 - adjust all other code segments where -Wswitch makes would claim that
   LIBXL_DOMAIN_TYPE_INVALID is not handled by adding a "default: abort();"
   clause.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Signed-off-by: Christoph Egger <Christoph.Egger@amd.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- use LIBXL_DOMAIN_TYPE_INVALID instead of -1 for
         libxl_domain_build_info.type's keyvar_init_val.
      -- what used to be libxl_primary_console_exec is now in
         libxl__primary_console_find so resolve the rejected hunk
         accordingly ]
Committed-by: Ian Campbell <ian.campbell@citrix.com>