net/vhost: retrieve vid for a given port

net/vhost: retrieve vid for a given port

In some cases when using the vHost PMD, certain vHost library functions
may still need to be accessed. One such example is the
rte_vhost_get_queue_num function which returns the number of virtqueues
reported by the guest - information which is not exposed by the PMD.

This commit introduces a new rte_eth_vhost function that returns the
'vid' associated with a given port id. This allows the PMD user to call
vHost library functions which require the 'vid' value.

Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Acked-by: John McNamara <john.mcnamara@intel.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>