net: fix -netdev socket,fd= for UDP sockets

This patch fixes -netdev socket,fd= for UDP sockets
Currently -netdev socket,fd=<...> results in

  qemu: error: specified mcastaddr "127.0.0.1" (0x7f000001) does not
    contain a multicast address
  qemu-system-x86_64: -netdev
    socket,id=n1,fd=3: Device 'socket' could not be initialized

To fix these we need to allow specifying multicast and fd arguments
for the same netdev. With this the user can specify "-netdev
fd=3,mcast=<IP:port>"

Cc: Jason Wang <jasowang@redhat.com>
Fixes: 3d830459b1eccdb61b75e2712fd364012ce5a115
Signed-off-by: Jens Freimann <jfreimann@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
