tracing: Tracing infrastructure

Add a new feature "tracing" that enables tracing functionality via the
"tracer" crate (sadly features and crates cannot share the same name.)

Setup: tracer::start()

The main functionality is a tracer::trace_scope()! macro that will add
trace points for the duration of the scope. Tracing events are per
thread.

Finish: tracer::end() this will write the trace file (pretty printed
JSON) to a file in the current directory.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

vmm/Cargo.toml

@@ -12,6 +12,7 @@ guest_debug = ["kvm"]
 kvm = ["hypervisor/kvm", "vfio-ioctls/kvm", "vm-device/kvm", "pci/kvm"]
 mshv = ["hypervisor/mshv", "vfio-ioctls/mshv", "vm-device/mshv", "pci/mshv"]
 tdx = ["arch/tdx", "hypervisor/tdx"]
+tracing = ["tracer/tracing"]
 
 [dependencies]
 acpi_tables = { path = "../acpi_tables" }
@@ -42,6 +43,7 @@ serde_json = "1.0.85"
 serial_buffer = { path = "../serial_buffer" }
 signal-hook = "0.3.14"
 thiserror = "1.0.35"
+tracer = { path = "../tracer" }
 uuid = "1.1.2"
 versionize = "0.1.6"
 versionize_derive = "0.1.4"