uhci: Add a completions_only flag for async completions

uhci: Add a completions_only flag for async completions

Add a completions_only flag, and set this when running process_frame for async
completion handling, this fixes 2 issues in a single patch:

1) It makes sure async completed packets get written to guest mem immediately,
even if all the bandwidth for the frame was consumed from the timer run
process_frame. This is necessary as delaying their writeback to the next frame
can cause the completion to get lost on migration.

2) The calling of process_frame from a bh on async completion causes iso
tds to get server more often they should, messing up usb sound class device
timing. By only processing completed packets, the iso tds get skipped fixing
this.

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