This patch adds IDs to usb packets.  Those IDs are (a) supposed to be
unique for the lifecycle of a packet (from packet setup until the packet
is either completed or canceled) and (b) stable across migration.

uhci, ohci, ehci and xhci use the guest physical address of the transfer
descriptor for this.

musb needs a different approach because there is no transfer descriptor.
But musb also doesn't support pipelining, so we have never more than one
packet per endpoint in flight.  So we go create an ID based on endpoint
and device address.

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