usb-linux: add hostport property

usb-linux: add hostport property

This patch adds a hostport property which allows to specify the host usb
devices to pass through by bus number and physical port.  This means you
can basically hand over one (or more) of the usb plugs on your host to
the guest and whatever device is plugged in there will show up in the
guest.

Usage:

  -device usb-host,hostbus=1,hostport=1

You can figure the port numbers by plugging in some usb device, then
find it in "info usbhost" and pick bus and port specified there.

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