hw/cxl/cxl-mailbox-utils: Add support for feature commands (8.2.9.6)

hw/cxl/cxl-mailbox-utils: Add support for feature commands (8.2.9.6)

CXL spec 3.1 section 8.2.9.6 describes optional device specific features.
CXL devices supports features with changeable attributes.
Get Supported Features retrieves the list of supported device specific
features. The settings of a feature can be retrieved using Get Feature and
optionally modified using Set Feature.

Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Reviewed-by: Fan Ni <fan.ni@samsung.com>
Signed-off-by: Shiju Jose <shiju.jose@huawei.com>
Link: https://lore.kernel.org/r/20240223085902.1549-2-shiju.jose@huawei.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <20240705123039.963781-3-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>