libxl: internal convenience macros

libxl: internal convenience macros

Provide some macros which are useful shorthands for use within libxl:
  * GC_INIT to initialise a gc from a ctx and GC_FREE to free it
  * CTX(gc) to give you back the ctx
  * LIBXL_TAILQ_INSERT_SORTED for inserting things into sorted lists

These will be used by later patches.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>