Introduce xentrace-perfetto tool

[TSnake41]

Add xentrace-perfetto as a Rust tool that can turn xentrace output into Perfetto traces that can then be analyzed through Perfetto in particular with the Web UI.

While testing, it takes a few seconds a process a medium sized xentrace binary (150-450 MB) covering ~15 seconds.

Tested and works good so far on XCP-ng and upstream Xen (but only tried on Intel CPUs for the time being).

How to use :

# On the XCP-ng/Xen host
xl info | grep cpu_mhz
# While doing something worth tracing (performance test, live migration, ...)
# Start xentrace (here for 10 seconds, can be stopped with ^C)
xentrace -T 10 trace.bin

# Elsewhere (or on the host itself) (a wrong cpu_mhz will cause wrong time)
xentrace-perfetto --cpu-mhz <cpu_mhz from xl> trace.bin trace.pftrace
# Then bring the pftrace file to https://ui.perfetto.dev/