convert net_init_socket() to NetClientOptions

convert net_init_socket() to NetClientOptions

I "reverse engineered" the following permissions between the -socket
sub-options:

             fd  listen  connect  mcast  udp | localaddr
  fd         x   .       .        .      .   | .
  listen     .   x       .        .      .   | .
  connect    .   .       x        .      .   | .
  mcast      .   .       .        x      .   | x
  udp        .   .       .        .      x   | x
  -------------------------------------------+
  localaddr  .   .       .        x      x     x

I transformed the code accordingly. The real fix would be to embed "fd",
"listen", "connect", "mcast" and "udp" in a separate union. However
OptsVisitor's enum parser only supports the type=XXX QemuOpt instance as
union discriminator.

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>