qmp: Add qom-list-properties to list QOM object properties

qmp: Add qom-list-properties to list QOM object properties

There is already 'device-list-properties' which does most of the job,
however it does not handle everything returned by qom-list-types such
as machines as they inherit directly from TYPE_OBJECT and not TYPE_DEVICE.
It does not handle abstract classes either.

This adds a new qom-list-properties command which prints properties
of a specific class and its instance. It is pretty much a simplified copy
of the device-list-properties handler.

Since it creates an object instance, device properties should appear
in the output as they are copied to QOM properties at the instance_init
hook.

This adds a object_class_property_iter_init() helper to allow class
properties enumeration uses it in the new QMP command to allow properties
listing for abstract classes.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Message-Id: <20180301130939.15875-3-aik@ozlabs.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>