Add serial_num and connection_id, clean up sr_dev_inst_new()

Since we want to uniquely identify devices in PulseView, we need
to have more information about the device than just vendor/model/
version. These two fields will be used to allow PV to access the
device's serial number and connection id.

The connection id can be pretty much anything as long as it's a
system-wide unique identifier. It can be a USB port path, an IP
address, a file name or whatever else.