When running "qemu-system-x86_64 -M pc,help" I noticed that some
properties were still missing their description. Add them now so
that users get at least a slightly better idea what they are all
about.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20211206134255.94784-1-thuth@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>