percpu.hh: Fix pointer arithmetic

Fixes the following complaint by GCC:

  In file included from ../../include/osv/per-cpu-counter.hh:13:0,
                   from ../../include/osv/trace-count.hh:10,
                   from api/trace.cc:20:
  ../../include/osv/percpu.hh: In instantiation of ‘T* dynamic_percpu<T, align>::addr(void*) [with T = long unsigned int; long unsigned int align = 8ul]’:
  ../../include/osv/percpu.hh:85:19:   required from ‘void dynamic_percpu<T, align>::construct() [with T = long unsigned int; long unsigned int align = 8ul]’
  ../../include/osv/percpu.hh:68:56:   required from ‘dynamic_percpu<T, align>::dynamic_percpu() [with T = long unsigned int; long unsigned int align = 8ul]’
  ../../include/osv/per-cpu-counter.hh:18:7:   required from here
  ../../include/osv/percpu.hh:88:37: warning: pointer of type ‘void *’ used in arithmetic [-Wpointer-arith]
           return static_cast<T*>(base + _offset);

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
setup.py: fix regression on Ubuntu 14.04/Fedora 20

This patch fixes a "no attribute" error:

Traceback (most recent call last):
  File "./scripts/setup.py", line 98, in <module>
    if dver.ec2_post_install:
AttributeError: type object 'Ubuntu_14_04' has no attribute 'ec2_post_install'

Signed-off-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>