torture: Avoid torture-test reboot loops

Currently, the various torture tests sometimes react to an early-boot
bug by rebooting.  This is almost always counterproductive, needlessly
consuming CPU time and bloating the console log.  This commit therefore
adds the "-no-reboot" argument to qemu so that reboot requests will
cause qemu to exit.

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