pci-assign: assignment should fail if we can't read config space

assigned_initfn()
  get_real_device()
    read()

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

hw/i386/kvm/pci-assign.c

@@ -576,6 +576,7 @@ again:
             goto again;
         }
         error_report("%s: read failed, errno = %d", __func__, errno);
+        return 1;
     }
 
     /* Restore or clear multifunction, this is always controlled by qemu */