pci-testdev: add optional memory bar

pci-testdev: add optional memory bar

Add memory bar to pci-testdev.  Size is configurable using the membar
property.  Setting the size to zero (default) turns it off.  Can be used
to check whether guests handle large pci bars correctly.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>