golang/xenlight: Errors are negative

golang/xenlight: Errors are negative

Commit 871e51d2d4 changed the sign on the xenlight error types (making
the values negative, same as the C-generated constants), but failed to
flip the sign in the Error() string function.  The result is that
ErrorNonspecific.String() prints "libxl error: 1" rather than the
human-readable error message.

Get rid of the whole issue by making libxlErrors a map, and mapping
actual error values to string, falling back to printing the actual
value of the Error type if it's not present.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Nick Rosbrook <rosbrookn@ainfosec.com>