sfc: explain that "attached" VFs only refer to Xen

During SRIOV disabling it is checked wether any VF is currently attached
to a guest, using pci_vfs_assigned function. However, this check only
works with VFs attached with Xen, not with vfio/KVM. Added comments
clarifying this point.

Also, replaced manual check of PCI_DEV_FLAGS_ASSIGNED flag and used the
helper function pci_is_dev_assigned instead.

Signed-off-by: Íñigo Huguet <ihuguet@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>