libxl: use LIBXL__LOG in libxl_ctx_alloc

Coverity points out that using LOG macro dereferences gc which is NULL
at that point. Use LIBXL__LOG instead.

CID: 1343291

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>