sched_credit: Raise bar for inter-socket migrations on mostly-idle systems

sched_credit: Raise bar for inter-socket migrations on mostly-idle systems

The credit scheduler ties to keep work balanced, even on a mostly idle
system.  Unfortunately, if you have one VM burning cpu and another VM
idle, the effect is that the busy VM will flip back and forth between
sockets.

This patch addresses this, by only migrating to a different socket if
the number of idle processors is twice that of the socket the vcpu is
currently on.

This will only affect mostly-idle systems; as the system becomes more
busy, other load-balancing code will come into effect.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>