examples/vmdq: fix Tx queue id

This patch fixes a core id issue in sample vmdq, in case core mask
doesn't start with lcore_id 0 but 20, for instance,
queue id should use core_id instead of lcore_id.

Signed-off-by: Ouyang Changchun <changchun.ouyang@intel.com>
Tested-by: Waterman Cao <waterman.cao@intel.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>