audio streaming from usb devices

I have streaming audio devices working within qemu-kvm. This is a port
of the changes to qemu.

Streaming audio generates a series of isochronous requests that are
repetitive and time sensitive. The URBs need to be submitted in
consecutive USB frames and responses need to be handled in a timely manner.

Summary of the changes for isochronous requests:

1. The initial 'valid' value is increased to 32. It needs to be higher
than its current value of 10 since the host adds a 10 frame delay to the
scheduling of the first request; if valid is set to 10 the first
isochronous request times out and qemu cancels it. 32 was chosen as a
nice round number, and it is used in the path where a TD-async pairing
already exists.

2. The token field in the TD is *not* unique for isochronous requests,
so it is not a good choice for finding a matching async request. The
buffer (where to write the guest data) is unique, so use that value instead.

3. TD's for isochronous request need to be completed in the async
completion handler so that data is pushed to the guest as soon as it is
available. The uhci code currently attempts to process complete
isochronous TDs the next time the UHCI frame with the request is
processed. The results in lost data since the async requests will have
long since timed out based on the valid parameter. Increasing the valid
value is not acceptable as it introduces a 1+ second delay in the data
getting pushed to the guest.

4. The frame timer needs to be run on 1 msec intervals. Currently, the
expire time for the processing the next frame is computed after the
processing of each frame. This regularly causes the scheduling of frames
to shift in time. When this happens the periodic scheduling of the
requests is broken and the subsequent request is seen as a new request
by the host resulting in a 10 msec delay (first isochronous request is
scheduled for 10 frames from when the URB is submitted).

[ For what's worth a small change is needed to the guest driver to have
more outstanding URBs (at least 4 URBs with 5 packets per URB).]

Signed-off-by: David Ahern <daahern@cisco.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
