microblaze: Fix latent bug with default DTB lookup

microblaze: Fix latent bug with default DTB lookup

microblaze_load_kernel() fails to call
qemu_find_file(QEMU_FILE_TYPE_BIOS, dtb_filename) when no -machine
options are given.  This can't normally happen, because -machine
option kernel is mandatory for this target.  Fix it anyway, by using
qemu_get_machine_opts().

Cc: Peter Crosthwaite <peter.crosthwaite@petalogix.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-id: 1372943363-24081-6-git-send-email-armbru@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>