[hdctools] servod: Add smarts to find unique servo USB device

BUG=chrome-os-partner:5502
TEST=manual, launch servod in w/ various servo's connected
1. default vid:pid of 0x403:6011
2. google vid:pid 0x18d1:5001
3. multiple devices of 1. & 2.
4. no devices
5. w/ and w/o cmdline switches of -v,-d,-s passed to servod.

Change-Id: I905b7d24cbabe31e45e096991e05b7954beaff69
Reviewed-on: http://gerrit.chromium.org/gerrit/6133
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Tested-by: Todd Broch <tbroch@chromium.org>
3 files changed