doc: describe --builtin-net-driver option in vhost app
doc: describe --builtin-net-driver option in vhost app

Very simple version of vhost-user driver in vhost sample will be used if
builtin-net-driver option is enabled. This driver is based on generic
vhost lib APIs. Unfortunately, the implementation is incompatible with
QEMU as protocol feature is not supported.

Signed-off-by: Marvin Liu <d89ea4150d91f674af88bbc382d184dd42ea906c@intel.com>
Acked-by: John McNamara <af31595c36a4239b9676e9b3a777d3e2f15a6c4d@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk