Add operation_mode for create_vhost_user_if, modify_vhost_user_if,
and sw_interface_vhost_user_details. Only polling mode is supported
for these APIs. Other mode is rejected and gets
VNET_API_ERROR_UNIMPLEMENTED error.
Change-Id: I0596f4e2c087aa2b6f78eb3e0b63910b1859641e Signed-off-by: Steven <sluong@cisco.com>