MFC 247870:

    Remove the virtio dependency entry for the VirtIO device drivers. This
    will prevent the kernel from linking if the device driver are included
    without the virtio module. Remove pci and scbus for the same reason.

    Also explain the relationship and necessity of the virtio and virtio_pci
    modules. Currently in FreeBSD, we only support VirtIO PCI, but it could
    be replaced with a different interface (like MMIO) and the device
    (network, block, etc) will still function.

Approved by:	grehan (mentor)