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.

Requested by:	luigi
Approved by:	grehan (mentor)
MFC after:	3 days