net: Do not fill legacy info_str for backends

As we use QAPI NetClientState->stored_config to store and get information
about backend network devices, we can drop fill of legacy field info_str
for them.

We still use info_str field for NIC and hubports, so we can not completely
remove it.

Signed-off-by: Alexey Kirillov <lekiravi@yandex-team.ru>
Signed-off-by: Jason Wang <jasowang@redhat.com>