Bug 1329289

A rebase can easily introduce unwanted changes to stable interfaces.
To avoid that, we need to find and review interface differences.

Machine type and device configuration is one such interface.  We need
to review:

- Semantics of machine types

  Machine types set configuration variables, including qdev
  compat_props.  Any new variables?  Lost ones?  Replaced ones?
  Changed values?

  This is entirely manual.

- Devices

  Any new devices?  Lost ones?

  Unwanted devices need to be cut or made inaccessible.

  Finding available devices manually is too error-prone.  Use
  introspection.  In modern QEMU: qom-list-types.

- Device properties

  Any new devices?  Lost ones?

  Finding available properties manually even more error-prone than
  devices.  In modern QEMU: device-list-properties.  Note that it
  doesn't work for some devices.

- Changed property types

  This should be rare.  The type returned by device-list-properties
  should do.

- Changed property default values

  We lack a way to introspect the default value.

  Long term, upstream introspection should be extended to cover this.

  Short term, we may have to settle for a downstream hack.

  Note that default values may depend on the machine type.

The checking is the same for major and minor releases.  The difference
is in the kind of changes we accept.

The review should produce a document detailing the changes.  For each
change, explain whether its wanted or not, and why.  For unwanted
changes, point to a BZ.

We need tooling to automate gathering the data to review.