After Commit 3499ba8198cad ("xen: Fix event channel callback via
INTX/GSI"), xenbus_probe() will be called too early on Arm. This will
recent to a guest hang during boot.

If the hang wasn't there, we would have ended up to call
xenbus_probe() twice (the second time is in xenbus_probe_initcall()).

We don't need to initialize xenbus_probe() early for Arm guest.
Therefore, the call in xen_guest_init() is now removed.

After this change, there is no more external caller for xenbus_probe().
So the function is turned to a static one. Interestingly there were two
prototypes for it.

Cc: stable@vger.kernel.org
Fixes: 3499ba8198cad ("xen: Fix event channel callback via INTX/GSI")
Reported-by: Ian Jackson <iwj@xenproject.org>
Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Link: https://lore.kernel.org/r/20210210170654.5377-1-julien@xen.org
Signed-off-by: Juergen Gross <jgross@suse.com>