While vcpu_id isn't necessarily a bad choice as an identifier for
the currently running vcpu, it is provided by userspace, and there
is close to no guarantee that it would be unique.

Switch it to vcpu_idx instead, for which we have much stronger
guarantees.
Reviewed-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230927090911.3355209-7-maz@kernel.orgSigned-off-by: Oliver Upton <oliver.upton@linux.dev>