svtrace only work when vhost-net is used

[ycongal-smile]

Because of https://github.com/seapath/svtrace/blob/main/svtracing/svtrace.py#L75C21-L75C28:

vhost_pid = get_pid("vhost")

svtrace only work when there is a vhost process on the hypervisor. i.e when the VM are linked to the physical network using vhost. One interesting usage of svtrace would be to compare the different ways to connect VMs : XDP, DPDK, bridges, SR-IOV, ...

One way to handle this would to gracefully handle (i.e not crash) cases where samples can't be timed and handle the missing data in the subsequent data processing (e.g, can't compute intermediate timing but only the end-to-end timing)