usb-linux: fix device path aka physical port handling

usb-linux: fix device path aka physical port handling

The device path isn't just a number.  It specifies the physical port
the device is connected to and in case the device is connected via
usb hub you'll have two numbers there, like this: "5.1".  The first
specifies the root port where the hub is plugged into, the second
specifies the port number of the hub where the device is plugged in.
With multiple hubs chained the string can become longer.

This patch renames devpath to port and makes it a string.   It also
adapts the sysfs parsing code accordingly.  The parser code is also more
strict now and skips the root hubs (which can't be assigned anyway).

The "info usbhost" monitor command now prints bus number, (os-assigned)
device address and physical port for each device.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>