xen/pvhsim: fix cpu onlining

xen/pvhsim: fix cpu onlining

Since commit 8d3c326f6756d1 ("xen: let vcpu_create() select processor")
the initial processor for all pv-shim vcpus will be 0, as no other cpus
are online when the vcpus are created. Before that commit the vcpus
would have processors set not being online yet, which worked just by
chance.

When the pv-shim vcpu becomes active it will have a hard affinity
not matching its initial processor assignment leading to failing
ASSERT()s or other problems depending on the selected scheduler.

Fix that by doing the affinity setting after onlining the cpu but
before taking the vcpu up. For vcpu 0 this is still in
sched_setup_dom0_vcpus(), for the other vcpus setting the affinity
there can be dropped.

Fixes: 8d3c326f6756d1 ("xen: let vcpu_create() select processor")
Reported-by: Sergey Dyasli <sergey.dyasli@citrix.com>
Tested-by: Sergey Dyasli <sergey.dyasli@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>