Trace every libvirt call in the KVM provider at V(1), visible in the integration test

A tracingAPI decorator wraps the 17-method libvirt seam and logs one
"libvirt call" line per RPC after it returns: method, the identifiers it
was given, curated results, duration, and on failure the error with its
libvirt code, name and message. At V(2) the same line carries the domain
and volume XML, metadata and addresses; upload bodies are never logged.
The pool applies it in call() when V(1) is enabled and logs connect,
dial-failure and drop events, taking the caller's logger explicitly like
every other helper in the package.

The integration test installs a testr logger at KVM_TEST_LOG_LEVEL
(default 1) so the trace interleaves with t.Log output, which also ends
the "log.SetLogger(...) was never called" warning. Docs gained a Logging
section and the test env var; the KVM provider execution log records run
4 (guest hang at GRUB) that prompted this.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/kvm-provider.md

@@ -186,6 +186,27 @@ replaces the VM).
 - `virsh domifaddr proxy-abc… --source arp` / `--source agent` shows what
   the operator sees when it looks for the IP.
 
+## Logging
+
+The provider logs through controller-runtime's logger, named `kvm`, with
+`provider`, `host` and `name` (the domain) on every line.
+
+- `--zap-log-level=debug` (logr V(1)): one line per provider operation
+  (`domain defined`, `KVM domain ensured running`, ...) **and one
+  `libvirt call` line per libvirt RPC**, named `kvm.libvirt`, with
+  `method`, the identifiers it was given (`domain`, `pool`, `volume`),
+  curated results (`state`, `path`, `interfaces`), `duration`, and on
+  failure `error` plus `libvirtCode`, `libvirtCodeName` and
+  `libvirtMessage`. Connection events (`libvirt host connected`, dial
+  failures, `libvirt connection dropped` with a reason) log at the same
+  level. A Create is about ten RPCs; a Get is four.
+- `--zap-log-level=2`: the same lines also carry the domain and volume
+  `xml`, the `metadata` element, the interface `addresses` and the
+  domain `names` from a listing.
+- Cloud-init is never logged at any level: the seed ISO upload reports
+  only `length` and `duration`. There is no raw wire tier like the GCP
+  provider's V(5); go-libvirt exposes no hook for it.
+
 ## Known gaps
 
 - **No quota or capacity detection.** libvirt reports an out-of-space
@@ -211,4 +232,6 @@ replaces the VM).
 
 Unit tests run over an in-memory fake of the libvirt API. The optional
 integration test against a real host is described in
-[testing.md](testing.md#the-kvm-integration-test-real-hypervisor).
+[testing.md](testing.md#the-kvm-integration-test-real-hypervisor); its
+`-v` output includes the `libvirt call` trace above, at the verbosity
+`KVM_TEST_LOG_LEVEL` selects (default 1).