xen-pciback: reconfigure also from backend watch handler

When multiple PCI devices get assigned to a guest right at boot, libxl
incrementally populates the backend tree. The writes for the first of
the devices trigger the backend watch. In turn xen_pcibk_setup_backend()
will set the XenBus state to Initialised, at which point no further
reconfigures would happen unless a device got hotplugged. Arrange for
reconfigure to also get triggered from the backend watch handler.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Cc: stable@vger.kernel.org
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/2337cbd6-94b9-4187-9862-c03ea12e0c61@suse.com


Signed-off-by: Juergen Gross <jgross@suse.com>