iio_readdev: Disable XML backend
iio_readdev: Disable XML backend

It makes no sense to have a way to use the XML backend there,
as this backend does not provide the low-level functions needed
to read a stream from a device.

Signed-off-by: Paul Cercueil <73d292422978d45c9eeef455bb1b24a0db453722@analog.com>

analogdevicesinc/libiio,analogdevicesinc/libiio,Sunderfield/libiio,sensarliar/libiio,gburca/libiio,analogdevicesinc/libiio,sensarliar/libiio,Sunderfield/libiio,gburca/libiio,sensarliar/libiio,analogdevicesinc/libiio,Sunderfield/libiio,gburca/libiio