xen: credit2: don't let b_avgload go negative.

xen: credit2: don't let b_avgload go negative.

The ASSERT() made effective by b5b5876619bd8ec2e
("xen: credit2: fix two s_time_t handling issues
in load balancing") triggers for b_avgload (spotted
by OSSTest).

b_avgload is where we store the prediction of how
the load of a runqueue will look like in the medium
to long term, because of a vcpu being added to or
removed from there.

On vcpu removal, saturate down b_avgload to zero,
as it makes very few sense to predict that the
load of a runqueue will at some point become negative!

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>