cpu: suffix cpu address spaces with cpu index

Renaming cpu address space names so that they won't be the same when
there are more than one.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20171123092333.16085-4-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>