The current description says that these options will create a device
on the IDE bus, which is only true on x86. So rephrase these sentences
a little bit to speak of "default bus" instead.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>