file-posix: Add dynamic-auto-read-only QAPI feature

In commit 23dece19 ('file-posix: Make auto-read-only dynamic') ,
auto-read-only=on changed its behaviour in file-posix for the 4.0
release. This change cannot be detected through the usual mechanisms
like schema introspection. Add a new feature flag to the schema to
allow libvirt to detect the presence of the new behaviour.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20190606153803.5278-7-armbru@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Comment tweaked on Eric Blake's advice]
Signed-off-by: Markus Armbruster <armbru@redhat.com>