libxl: add convenience macros to qmp_send() in libxl_qmp.c

libxl: add convenience macros to qmp_send() in libxl_qmp.c

Update qmp_send() in libxl_qmp.c to use the new convenience macros
declared in libxl_internal.h. Uses GC_INIT at the top of the function,
and GC_FREE at the exit. Since GC_INIT returns a libxl__gc by reference
and not by value, remove the address operator from the left of the
variable gc where it is passed as a parameter.

Suggested-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Kelley Nielsen <kelleynnn@gmail.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>