xen/blkback: Change statistics counter types to unsigned

These values shouldn't be negative, but after an overflow their value
can turn into negative, if they are signed. xentop can show bogus
values in this case.

Signed-off-by: Zoltan Kiss <zoltan.kiss@citrix.com>
Reported-by: Ichiro Ogino <ichiro.ogino@citrix.co.jp>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>