- Remove two write-only local variables
  - Remove unused element in the vtnet_rxq structure

MFC after:	1 week
Remove kernel specific macro out of the VirtIO PCI header file

The eventual goal is to share this file with userland, so
remove the macro that is only specific for virtio_pci(4).
Instead, add the VIRTIO_PCI_CONFIG_OFF macro from Linux to
get the config size whether MSIX is enabled or not.

MFC after:	1 week