libxl: abolish libxl_ctx_postfork

libxl: abolish libxl_ctx_postfork

libxl's task has become too complicated (particularly in the presence
of both forking and multithreading) to support reuse of the same
libxl_ctx after fork.

So abolish libxl_ctx_fork.  xl instead simply initialises a new
libxl_ctx.

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>