[xwl] Fix xcb to wayland clipboard

A seemingly unrelated change to cursors broke xwayland's clipboard
syncing, presumably because of a change in how startup is ordered with
regards to syncing between our internal wayland connection causing a
crash.

It was fixed with a flush, but ultimately guarding things coming up in
the natural order works better.

Long term plans are to finish removing the internal connection.

See xwayland_selections_test in autotests.