iowatcher: spawn NPROCESSORS_ONLN for rsvg-convert-s

iowatcher: spawn NPROCESSORS_ONLN for rsvg-convert-s

iowatcher currently always spawns 8 rsvg-convert processes, no matter
how many CPUs a system has.  I did some limited testing of different
numbers of rsvg-convert processes.  Here are the results:

8 processes:
real	4m2.194s
user	23m36.665s
sys	0m38.523s

20 processes:
real	2m28.935s
user	24m51.817s
sys	0m49.227s

40 processes:
real	2m28.150s
user	24m56.994s
sys	0m49.621s

Note that this is the time it takes for a full run of iowatcher -- I
didn't separate out just the rsvg-convert portion.

Given the above results, it seems like a reasonable thing to spawn one
rsvg-convert process per cpu.

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>