error: Don't abuse qemu_error() for non-error in qbus_find()

error: Don't abuse qemu_error() for non-error in qbus_find()

qbus_find() adds an informational line to error messages, and prints
both lines with one qemu_error().  Use error_printf() for the
informational line instead.

While there, simplify: instead of printing buffers filled by
qbus_list_bus() and qbus_list_dev() in one go, make them print it.