rcutorture: Add --no-initrd argument to kvm.sh

This commit adds the --no-initrd argument to kvm.sh, which permits
initrd to be contained in a root partition specified by the --bootargs
argument.  Without --no-initrd, the kernel build expects an initrd
directory in the same rcutorture directory that contains bin and configs.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Greg KH <gregkh@linuxfoundation.org>