torture: Accept time units on kvm.sh --duration argument

The "--duration <minutes>" has worked well for a very long time, but
it can be inconvenient to compute the minutes for (say) a 28-hour run.
It can also be annoying to have to let a simple boot test run for a full
minute.  This commit therefore permits an "s" suffix to specify seconds,
"m" to specify minutes (which remains the default), "h" suffix to specify
hours, and "d" to specify days.

With this change, "--duration 5" still specifies that each scenario
run for five minutes, but "--duration 30s" runs for only 30 seconds,
"--duration 8h" runs for eight hours, and "--duration 2d" runs for
two days.

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