torture: Don't redirect qemu-cmd comment lines

Currently, kvm-test-1-run-qemu.sh applies redirection to each and every
line of each qemu-cmd script.  Only the first line (the only one that
is not a bash comment) needs to be redirected.  Although redirecting
the comments is currently harmless, just adding to the comment, it is
an accident waiting to happen.  This commit therefore adjusts the "sed"
command to redirect only the qemu-system* command itself.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>