nfs-iostat.py: don't wait for an extra interval when given a count

If I invoke the tool with an interval of 10 and a count of 2, it will:
- show the summary
- sleep 10s
- show the stats for the last 10s
- sleep 10s
- exit

Signed-off-by: David Lecorfe <dlecorfec@gmail.com>
Signed-off-by: Steve Dickson <steved@redhat.com>