serial_hid: make --list-serial output compatible with conn=, print "VID.PID"

serial_hid: make --list-serial output compatible with conn=, print "VID.PID"

When the list of all connections gets created which are supported by the
HID serial transport, items contain a "hid/ch9325/raw=/dev/hidraw3" path
and a "1a86:e008" pair of vendor and product IDs.

Separate the VID/PID pair by a period not a colon, so that --list-serial
output immediately becomes usable with "--driver <name>:conn=<spec>"
invocations. Eliminate the necessity to adjust clipboard context by the
user. This improves usability in cases where not a single connection
gets addressed, but a group of connections gets specified by ambiguous
conn= specs.

$ sigrok-cli -d uni-t-ut32x:conn=1a86.e008 --scan