Check for presence of disks in vmstat

Check for presence of disks in vmstat

vmstat -d or vmstat -p would crash mysteriously under different
circumstances. The problem was eventually tracked down to /sys not
being mounted which meant is_disk() always returned false.
The partition would then be attempted to be linked to a non-existent
disk causing a segfault.

vmstat will now not link to a disk if none exists.
The change in testing will skip those tests when /sys/block doesn't
exist.

Many thanks to Daniel Schepler for his analysis and suggestions.

Bug-Debian: http://bugs.debian.org/736628