libxl: init: Provide a gc later in libxl_ctx_alloc

libxl: init: Provide a gc later in libxl_ctx_alloc

Provide libxl__gc *gc for the second half of libxl_ctx_alloc.
(For the first half of the function, gc is in scope but set to NULL.)

This makes it possible to make gc-requiring calls.  For example, it
makes error logging more convenient.

Make use of this by changing the logging calls to use the LOG*
convenience macros.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>