Reorganization of struct NetClientOptions (commit e4ba22b3) caused a
compilation failure of the netmap backend. This patch fixes the issue
by properly accessing the union field.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Vincenzo Maffione <v.maffione@gmail.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>