xen/vm-event: Fix interactions with the vcpu list

xen/vm-event: Fix interactions with the vcpu list

vm_event_resume() should use domain_vcpu(), rather than opencoding it
without its Spectre v1 safety.

vm_event_wake_blocked() can't ever be invoked in a case where d->vcpu is
NULL, so drop the outer if() and reindent, fixing up style issues.

The comment, which is left alone, is false.  This algorithm still has
starvation issues when there is an asymetric rate of generated events.

However, the existing logic is sufficiently complicated and fragile that
I don't think I've followed it fully, and because we're trying to
obsolete this interface, the safest course of action is to leave it
alone, rather than to end up making things subtly different.

Therefore, no practical change that callers would notice.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Razvan Cojocaru <rcojocaru@bitdefender.com>