Update check_iostat.pl

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#11/08/2014 - Another way to work around this is to add sudo privilege for 
#             nrpe user (nagios most of the time in rpm packages), by adding 
#             the following line in /etc/sudoers
#                 nagios ALL = NOPASSWD: /usr/sbin/pvs, /sbin/multipath
#             You might have to comment out this line
#                 #Default requiretty
#             if you receive the following error
#                 "sudo: sorry, you must have a tty to run sudo"
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#12/08/2014 - Corrected the regex for disk grouping in VG for sdXY disks
#             Added a "all" or "vgonly" switch to display only VG. This is 
#             useful for LVM systems with a lot of disks, partitions, 
#             multipath devices where the graphics and perfdata become 
#             unusable really fast
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