net: Reinstate '-net nic, model=help' output as documented in man page

net: Reinstate '-net nic, model=help' output as documented in man page

While refactoring the NIC initialization code, I broke '-net nic,model=help'
which no longer outputs a list of available NIC models.

Fixes: 2cdeca04adab ("net: report list of available models according to platform")
Cc: qemu-stable@nongnu.org
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Jason Wang <jasowang@redhat.com>