xml: nodedev: add class info for pci capability

xml: nodedev: add class info for pci capability

This info can be useful to filter devices visible
to mgmt clients so that they won't see devices that
unsafe/not meaningful to pass thru.

Provide class info the way it is provided by udev or
kernel that is as single 6-digit hexadecimal.

Class element is not optional. I guess this should not
break users that use virNodeDeviceCreateXML because
they probably specify only scsi_host capability on
input and then node device driver gets other capabilities
from udev after device appeared.

HAL driver does not get support for the new element in
this patch.

Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>