xen: sched: deal with vCPUs being or becoming online or offline

xen: sched: deal with vCPUs being or becoming online or offline

If a vCPU is, or is going, offline we want it to be neither
assigned to a pCPU, nor in the wait list, so:
- if an offline vcpu is inserted (or migrated) it must not
  go on a pCPU, nor in the wait list;
- if an offline vcpu is removed, we are sure that it is
  neither on a pCPU nor in the wait list already, so we
  should just bail, avoiding doing any further action;
- if a vCPU goes offline we need to remove it either from
  its pCPU or from the wait list.

Signed-off-by: Dario Faggioli <dfaggioli@suse.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Message-Id: <156412235656.2385.13861979113936528474.stgit@Palanthas>