net: don't use set/get_pointer() in set/get_netdev()

Commit 1ceef9f27359cbe92ef124bf74de6f792e71f6fb (net: multiqueue
support) tries to use set_pointer() and get_pointer() to set and get
NICPeers which is not a pointer defined in DEFINE_PROP_NETDEV. This
trick works but result a unclean and fragile implementation (e.g
print_netdev and parse_netdev).

This patch solves this issue by not using set/get_pinter() and set and
get netdev directly in set_netdev() and get_netdev(). After this the
parse_netdev() and print_netdev() were no longer used and dropped from
the source.

Cc: Markus Armbruster <armbru@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>