QWaylandShmBuffer: use a memfd if we can for a simple memory buffer

which has a file descriptor we can also use to share with

Change-Id: I8d96dea9955d4c749b99fffd14cd61628c616b30
Reviewed-by: Johan Helsing <johan.helsing@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>