Don't use control requests to read device serial numbers.

Instead of sending control requests to all devices to read their serial
numbers, read the cached strings from sysfs. This fixes two problems:
1) The control requests sometimes timed out, causing devices to show up
as "????????????????", and 2) a control request could mess up devices
which were in the middle of being flashed.

Also enumerate by walking sysfs rather than devs so that we can avoid
opening host controller devices, which can be slow. On a system with 10
EHCI controllers, fastboot devices now takes 7 msec instead of 700 msec.

Change-Id: I2ad2d58b48268d7950bd279fd6a6753dc2f79514