torture: Make kvm.sh arguments accumulate

Given that kvm.sh in invoked from scripts, it is only natural for
different levels of scripting to provide their own Kconfig option values,
for example.  Unfortunately, right now, the last such argument on the
command line wins.

This commit therefore makes the --bootargs, --configs, --kconfigs,
--kmake-args, and --qemu-args argument values accumulate.  For example,
where "--configs TREE01 --configs TREE02" would previously have run only
scenario TREE02, now it will run both scenarios.

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