KVM: Implement barriers before accessing kvm->buses[] on SRCU read paths

KVM: Implement barriers before accessing kvm->buses[] on SRCU read paths

This ensures that, if a VCPU has "observed" that an IO registration has
occurred, the instruction currently being trapped or emulated will also
observe the IO registration.

At the same time, enforce that kvm_get_bus() is used only on the
update side, ensuring that a long-term reference cannot be obtained by
an SRCU reader.

Signed-off-by: Keir Fraser <keirf@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>