osc: add domain plugin_find_plot_with_domain() and fix bug

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
plugins/fmcomms2.c: don't use iio_device_get_channel()
Merge branch 'libiio-rc1' of https://github.com/analogdevicesinc/iio-oscilloscope into libiio-rc1
fmcomms5 WIP4

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>