usb: Add a speedmask to devices

This is used to indicate at which speed[s] the device can operate,
so that this can be checked to match the ports capabilities when it gets
attached to a bus.

Note that currently all usb1 emulated device claim to be fullspeed, this
seems to not cause any problems, but still seems wrong, because with real
hardware keyboards, mice and tablets usually are lo-speed, so reporting these
as fullspeed devices seems wrong.

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