hmp: Make json format optional for qom-set

hmp: Make json format optional for qom-set

Commit 7d2ef6dcc1cf ("hmp: Simplify qom-set") switched to the json
parser, making it possible to specify complex types. However, with this
change it is no longer possible to specify proper sizes (e.g., 2G, 128M),
turning the interface harder to use for properties that consume sizes.

Let's switch back to the previous handling and allow to specify passing
json via the "-j" parameter.

Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Daniel P. Berrangé" <berrange@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20200610075153.33892-1-david@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>