microblaze: fix memory leak

When not assign a -dtb argument, the variable dtb_filename
storage returned from qemu_find_file(), which should be freed
after use. Alternatively we define a local variable filename,
with 'char *' type, free after use.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>