Do not attempt to move the pointer if the widget is not yet realized.
The mouse cursor is placed to the corner of the screen, on X11 at least,
as x_root and y_root are then miscalculated. (this is not reproducible
on Wayland, because Gtk doesn't implement device warping there)

This also fixes the following warning at start:
qemu: Gdk: gdk_window_get_root_coords: assertion 'GDK_IS_WINDOW (window)' failed

Fixes: 6effaa16 ("ui: set cursor position upon listener
registration")
Reported-by: Bernhard Beschow <shentey@gmail.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Tested-by: Bernhard Beschow <shentey@gmail.com>
Message-Id: <20230320132624.1612464-1-marcandre.lureau@redhat.com>