Virtual session: Workaround for SR_CONF_CAPTUREFILE (#944)

Virtual session: Workaround for SR_CONF_CAPTUREFILE (#944)

This is a (hopefully temporary) workaround for the
SR_CONF_CAPTUREFILE mechanism. The value for
vdev->capturefile is set by this, however only once
via stream_session_data().
During stream processing in stream_session_data(),
capturefile may receive new values - e.g. when there
are multiple logic files or if there is analog data.
With that, the initially set capturefile is overwritten.

When re-loading the file, we are then running into
issues because we don't know what the initial value was.
As all .sr files use "logic-1" by default and, we
simulate the behavior of stream_session_data() and
assign this name to capturefile if there are logic
channels present.

With this change, all three kinds of files reload
as expected: logic only, analog only and mixed signal.
For this reason, it's a short-term fix for #944.