cpupools: Make interface more consistent

cpupools: Make interface more consistent

The current cpupools code interface is a bit inconsistent.  This
patch addresses this by making the interaction for each
vcpu in a pool look like this:

alloc_vdata() -- allocates and sets up vcpu data
insert_vcpu() -- the vcpu is ready to run in this pool
remove_vcpu() -- take the vcpu out of the pool
free_vdata()  -- delete allocated vcpu data

(Previously, remove_vcpu and free_vdata were combined into a "destroy
vcpu", and insert_vcpu was only called for idle vcpus.)

This also addresses a bug in credit2 which was caused by a
misunderstanding of the cpupools interface.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Juergen Gross <juergen.gross@ts.fujitsu.com>