ui/console: allocate ui_timer in QemuConsole

Although at this point only QemuGraphicConsole have hw_ops that
implements ui_info() callback, it makes sense to keep the code in the
base QemuConsole, to simplify conditions for the caller.

As of now, the code didn't reach a NULL timer because dpy_set_ui_info()
checks if dpy_ui_info_supported() (hw_ops->ui_info != NULL), which is
false for text_console_ops. This is a bit fragile, let simply allocate
and free the timer in the base class.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230830093843.3531473-26-marcandre.lureau@redhat.com>