vmxnet3: move to drivers/net/

vmxnet3: move to drivers/net/

Move vmxnet3 PMD to drivers/net directory.
As part of the move, rename the "vmxnet3" subdirectory, containing the
original FreeBSD drivers, from "vmxnet3" to the more standard name
"base", to indicate it contains the base drivers used for the
implementation.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: John McNamara <john.mcnamara@intel.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>