usbredirhost: new callback to drop isoc packets

For streaming devices it might be necessary from application to drop
data for different reasons. This patch provides a new callback that it
is called before queueing the most recent isoc packet.

usbredirhost uses the device information to set two levels of threshold.
In case application's buffer size is higher then the higher threshold we
will drop enough isoc packets to be able to provide a complete video
frame again.

The calculation aims at 10fps as worst case. The higher threshold is set
to 180ms and the lower threshold to 30ms which would give us at least
150ms of continuous data.

Related: https://bugzilla.redhat.com/show_bug.cgi?id=1264156



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