golang/xenlight: Don't leak memory on context open failure

golang/xenlight: Don't leak memory on context open failure

If libxl_ctx_alloc() returns an error, we need to destroy the logger
that we made.

Restructure the Close() method such that it checks for each resource
to be freed and then frees it.  This allows Close() to be come
idempotent, as well as to be a useful clean-up to a partially-created
context.

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