rcutorture/kvm.sh: Use consistent help text for --qemu-args

The '--qemu-args' option's help text is wrongly copied from '--qemu-cmd'
option and its argument type description message format is inconsistent
with other arguments.  This commit fixes the usage and type messages to
be consistent with others.

Fixes: e9ce6400 ("rcutorture: Add --qemu-args argument to kvm.sh")

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>