SysFSTools: change 'product' to 'device'

so it is more consistend with the source (it comes from the file
'device') as well as the subsytem_device field

the only place we use that field is in the same file in pci_dev_bind_to_vfio,
which we also change here, so that should not be a breaking change
(in qemu-server we only really use the existance and the has_fl_reset
flag)

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>