Ensure iostat plugin on sunos outputs correct type

This plugin may return a float. This makes RRD complain, visible on the
munin master as the following error:

 - 2013/11/07 09:40:07 [ERROR] In RRD: Error updating
   /var/lib/munin/example.com/box.example.com-iostat-diskr-d.rrd:
   /var/lib/munin/example.com/box.example.com-iostat-diskr-d.rrd: not a
   simple signed integer: '27304781710438.4'

This commit rounds the value to the nearest signed integer.