Currently we are submitting iso packets to the host one at a time, as we
receive them from the emulated host controller. This has 2 problems:
1) If we were fast enough to submit every packet in time for the next host host
controller usb frame, we would be generating 1000 hardware interrupts per
second on the host
2) We are not fast enough to submit every packet in time for the next host host
controller usb frame, causing us to not submit iso urbs in some usb frames
which causes devices with an endpoint with an interval of 1 ms (so every
frame) to loose data. This causes for example ubs-1.1 webcams to not work
properly (usb-2.0 is not supported at all atm).

This patch fixes both problems by changing the iso packet pass through handling
to buffer packets. This version only does so for iso input packets (webcams,
audio in) I'm working on a second patch extending this to iso output packets
(audio out).

This patch makes use of the linux batching of iso packets in one urb.
When an iso in packet gets received from the emulated host controller,
it immediately submits 3 urbs with 32 iso in packets each. This causes
the host to only get an hw interrupt every 32 packets dropping the
interrupt rate to 32 interrupts per second and gives it a queue of urbs
to work from once the first 32 iso in packets have been received to make sure
no packets are dropped.

Besides submitting a whole bunch or urbs as soon as the first urb is
received, effectively creating a buffer inside the kernel, this patch also
gets rid of the asynchroneous completion for iso in urbs. Instead they are
only marked as complete in the fd write callback (which usbfs uses to signal
complete urbs). These complete packets then get consumed by returning them
synchroneously to the emulated host controller when it submits an iso in
packet for the ep in question. When no complete packets are ready (which
happens when the stream is starting) a 0 length packet gets returned to
the emulated host controller.

With this patch I've several usb-1.1 webcams working well with usb pass
through, where as without this patch none of them work.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>