xen/arm: p2m: Perform local TLB invalidation on vCPU migration

xen/arm: p2m: Perform local TLB invalidation on vCPU migration

The ARM architecture allows an OS to have per-CPU page tables, as it
guarantees that TLBs never migrate from one CPU to another.

This works fine until this is done in a guest. Consider the following
scenario:
    - vcpu-0 maps P to V
    - vpcu-1 maps P' to V

If run on the same physical CPU, vcpu-1 can hit in TLBs generated by
vcpu-0 accesses, and access the wrong physical page.

The solution to this is to keep a per-p2m map of which vCPU ran the last
on each given pCPU and invalidate local TLBs if two vPCUs from the same
VM run on the same CPU.

Unfortunately it is not possible to allocate per-cpu variable on the
fly. So for now the size of the array is NR_CPUS, this is fine because
we still have space in the structure domain. We may want to add an
helper to allocate per-cpu variable in the future.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>