credit2: Add credit2 scheduler to hypervisor

credit2: Add credit2 scheduler to hypervisor

This is the core credit2 patch.  It adds the new credit2 scheduler to
the hypervisor, as the non-default scheduler.  It should be emphasized
that this is still in the development phase, and is probably still
unstable.  It is known to be suboptimal for multi-socket systems.

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