hw/xen: Fix xen_bus_realize() error handling

hw/xen: Fix xen_bus_realize() error handling

The Error ** argument must be NULL, &error_abort, &error_fatal, or a
pointer to a variable containing NULL.  Passing an argument of the
latter kind twice without clearing it in between is wrong: if the
first call sets an error, it no longer points to NULL for the second
call.

xen_bus_realize() is wrong that way: it passes &local_err to
xs_node_watch() in a loop.  If this fails in more than one iteration,
it can trip error_setv()'s assertion.

Fix by clearing @local_err.

Fixes: c4583c8c394e (xen-bus: reduce scope of backend watch)
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20250314143500.2449658-2-armbru@redhat.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>