[Backport to 3.34] wayland: preserve `xkb_state` on VT switch

Merged Olivier Fourdan requested to merge ofourdan/mutter:issue344-3-34 into gnome-3-34

On VT switch, the devices are removed, which means for Wayland disabling the keyboard.

When the keyboard is disabled, the associated xkb_state is freed and recreated whenever the keyboard is re-enabled when switching back to the compositor VT.

That means the xkb_state for Wayland is lost whereas the same for clutter is kept, which causes to a discrepancy with locked modifiers on VT switch.

To avoid that issue, preserve the XKB info only to dispose it when the keyboard is eventually finalized.

Closes: #344 (closed) !1185 (merged) (cherry picked from commit 5b30a52b)

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