hw/arm/xlnx-zynqmp.c: Avoid memory leak in error-return path

hw/arm/xlnx-zynqmp.c: Avoid memory leak in error-return path

In xlnx_zynqmp_realize() if the attempt to realize the SD
controller object fails then the error-return path will leak
the 'bus_name' string. Fix this by deferring the allocation
until after the realize has succeeded.

Fixes: Coverity CID 1421911
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20200324134947.15384-2-peter.maydell@linaro.org