fw_cfg-test: Fix test path to include architecture

Use qtest_add_func() instead of g_test_add_func() to reflect
the architecture tested, changing GTester paths as follows:

  /fw_cfg/foo -> /x86_64/fw_cfg/foo etc.

Cc: qemu-stable@nongnu.org
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>