xen, cpupools: Fix cpupool-move to make more consistent

xen, cpupools: Fix cpupool-move to make more consistent

The full order for creating new private data structures when moving
from one pool to another is now:
* Allocate all new structures
 - Allocate a new private domain structure (but don't point there yet)
 - Allocate per-vcpu data structures (but don't point there yet)
* Remove old structures
 - Remove each vcpu, freeing the associated data structure
 - Free the domain data structure
* Switch to the new structures
 - Set the domain to the new cpupool, with the new private domain
 structure
 - Set each vcpu to the respective new structure, and insert

This is in line with a (fairly reasonable) assumption in credit2 that
the private structure of the domain will be the private structure
pointed to by the per-vcpu private structure.

Also fix a bug, in which insert_vcpu was called with the *old* vcpu
ops rather than the new ones.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Committed-by: Keir Fraser <keir@xen.org>