torture: Permit kvm-again.sh --duration to default to previous run

Currently, invoking kvm-again.sh without a --duration argument results
in a bash error message.  This commit therefore adds quotes around the
$dur argument to kvm-transform.sh to allow a default duration to be
taken from the earlier run.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>