libxl: init: libxl__poller_init and _get take gc

libxl: init: libxl__poller_init and _get take gc

Change libxl__poller_init and libxl__poller__get to take a libxl__gc*
rather than a libxl_ctx*.  The gc is not used for memory allocation
but simply to provide the standard local variable "gc" expected by the
convenience macros.  Doing this makes the error logging more
convenient.

Hence, convert the logging calls to use the LOG* convenience macros.

And consequently, change the call sites, and the function bodies to
use CTX rather than ctx.

Also convert a call to malloc() (with error check) in
libxl__poller_get, to libxl__zalloc (no error check needed).

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>