Document newly added loader tunable and sysctl variables.
 o hw.vge.msi_disable
 o dev.vge.%d.int_holdoff
 o dev.vge.%d.rx_coal_pkt
 o dev.vge.%d.tx_coal_pkt

Document more VIA Velocity family controllers to vge(4). Previously
it mentioned only VT6122. While I'm here remove the mention of
VT3119 which seems to be VIA's internal model name and VT3119
wouldn't be available to end users.

Reviewed by:  brueffer