vpci/header: cope with devices not having vpci allocated

vpci/header: cope with devices not having vpci allocated

When traversing the list of pci devices assigned to a domain cope with
some of them not having the vpci struct allocated. It should be
possible for the hardware domain to have read-only devices assigned
that are not handled by vPCI, such support will be added by further
patches.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: ee045f3a4a6dddb09f5aa96a50cceaae97d3245f
master date: 2023-05-26 09:18:37 +0200