microblaze: boot.c: Don't try to find NULL file

Previously if no device tree was passed to microblaze_load_kernel() then
qemu_find_file() would try to find a NULL pointer. To avoid this put a
check around qemu_find_file().

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>