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/plans-executions/2026-09-04-1404-kvm-provider.md

@@ -462,3 +462,18 @@ lookup already selected the element by URI. The fake now strips the
 declaration when storing metadata, so the provider's Get and ListByTag
 tests parse the shape libvirt really returns, and a new
 `TestParseInstanceMetadata_libvirtBareForm` pins it.
+
+### Run 4: guest hangs at GRUB's "Booting" line
+
+Create and start passed; Get polled `Provisioning` with no IP while the
+VM sat at 100 % CPU after GRUB printed "Booting `Debian GNU/Linux`" and
+before the kernel wrote anything to the serial console. The domain XML
+matches a working VM on the same host (q35, BIOS, host-passthrough,
+virtio disk, SATA cdrom, br0), so the difference is the disk content:
+the cached image was downloaded without a checksum and never verified.
+Diagnosis was handed to the user (sha512 against Debian's published
+sums, `qemu-img check`, and a `virt-install --import` of the same image
+outside the operator). Because the provider's own logs were invisible in
+the test output, this run also produced a side plan:
+[2026-09-10-0805-kvm-libvirt-tracing](2026-09-10-0805-kvm-libvirt-tracing.md),
+which traces every libvirt RPC at V(1) and routes it into `-v` output.