libxl: change most remaining LIBXL_LOG to LOG in libxl_qmp.c

libxl: change most remaining LIBXL_LOG to LOG in libxl_qmp.c

Coding style has recently been changed for libxl. The convenience
macro LOG() has been introduced, and invocations of the old macro
LIBXL__LOG() are to be replaced with it. Change occurences of the
old macro to the new one in the functions qmp_handle_response()
and qmp_handle_error_response(). The new macros need access to a
local libxl__gc *gc, so add it as a parameter to both these functions,
and pass the instance in qmp_next() down the call chain to
qmp_handle_response() and in turn to qmp_handle_error_response().

Suggested-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Kelley Nielsen <kelleynnn@gmail.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
[ijc -- reverted one unintential w/s change]