"size" is a property of TYPE_MEMORY_BACKEND.
host_memory_backend_get_size() and host_memory_backend_set_size() use
visit_type_size().

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20170607163635.17635-39-marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>