gnttab: steal maptrack entries from other VCPUs

gnttab: steal maptrack entries from other VCPUs

If a guest is not evenly grant mapping across its VCPUs one of the
VCPUs may run out of free maptrack entries even though other VCPUs
have many free.

If this happens, "steal" free entries from other VCPUs.  We want to
steal entries such that:

a) We avoid ping-ponging stolen entries between VCPUs.

b) The number of free entries owned by each VCPUs tends (over time) to
   the number it uses.

So when stealing, we select a VCPU at random (reducing (a)) and we
transfer the stolen entries to the thief VCPU (aiming for (b)).

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>