[libvirt] [sandbox][PATCH] Typo and example fix

I fixed a typo in the strace debug feature, if you specified LIBVIRT_SANDBOX_STRACE=poll it would write "strace =poll" in the kernel command line and consequently it wouldn't get picked up because of that extra space. Also, the example virt-sandbox.py was a bit outdated so I updated it. The shell.py doesn't work as it has serial1 hardcoded for qemu. I haven't checked the rest.