qom: Fix object_property_add_alias() with [*]

Commit 80742642 (qom: Add description field in ObjectProperty struct)
introduced property descriptions and copied them for alias properties.

Instead of using the caller-supplied property name, use the returned
property name for setting the description. This avoids an Error when
setting a property description for a property with literal "[*]" that
doesn't exist due to automatic property naming in object_property_add().

Reviewed-by: Gonglei <arei.gonglei@huawei.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: qemu-stable@nongnu.org (v2.2+)
Signed-off-by: Andreas Färber <afaerber@suse.de>