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

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

qdev_device_help() prints device information with qemu_error().  A
later commit will make qemu_error() print additional stuff that is
only appropriate for proper errors, and then this will break.  Use
error_printf() instead.

While there, simplify: instead of printing a buffer filled by
qdev_print_devinfo() in one go, make qdev_print_devinfo() print it.