doc: clarify usage of netvsc PMD
doc: clarify usage of netvsc PMD

Since netvsc PMD does not support SR-IOV accelerated networking,
it is not recommended for use on Azure. Make this clear in the
guide.

Signed-off-by: Stephen Hemminger <6e2d829dccd9a999246d37fa28053b3b607441a0@microsoft.com>

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