docs/usecase: disable dpdk plugin in vpp for some cases

Disable DPDK plugin in VPP for memif and loopback connections to
skip DPDK interfaces enumeration.

Signed-off-by: Liangxing Wang <liangxing.wang@arm.com>
Reviewed-by: Yefeng Lin <yefeng.lin@arm.com>
Change-Id: I277af443e5b4167af5cbcaee972bd0273c78caa8