sched: fix sched_move_domain() for domain without vcpus

In case a domain is created with a cpupool other than Pool-0 specified
it will be moved to that cpupool before any vcpus are allocated.

This will lead to a NULL pointer dereference in sched_move_domain().

Fix that by tolerating vcpus not being allocated yet.

Fixes: 70fadc41 ("xen/cpupool: support moving domain between cpupools with different granularity")
Reported-by: Bertrand Marquis <bertrand.marquis@arm.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Dario Faggioli <dfaggioli@suse.com>

xen/common/sched/core.c

@@ -651,7 +651,7 @@ int sched_move_domain(struct domain *d, struct cpupool *c)
     struct scheduler *old_ops = dom_scheduler(d);
     void *old_domdata;
     unsigned int gran = cpupool_get_granularity(c);
-    unsigned int n_units = DIV_ROUND_UP(d->max_vcpus, gran);
+    unsigned int n_units = d->vcpu[0] ? DIV_ROUND_UP(d->max_vcpus, gran) : 0;
     int ret = 0;
 
     for_each_vcpu ( d, v )