qmp: Include "share" property of memory backends

qmp: Include "share" property of memory backends

Let's include the property, which can be helpful when debugging,
for example, to spot misuse of MAP_PRIVATE which can result in some ugly
corner cases (e.g., double-memory consumption on shmem).

Use the same description we also use for describing the property.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Eduardo Habkost <ehabkost@redhat.com> for memory backend and machine core
Cc: Eric Blake <eblake@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210510114328.21835-13-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>