Revert "Change net/socket.c to use socket_*() functions" again

This reverts commit 883e4f7624e10b98d16d9adaffb8b1795664d899.

This code changed net/socket.c from using socket()+connect(),
to using socket_connect(). In theory this is great, but in
practice this has completely broken the ability to connect
the frontend and backend:

  $ ./x86_64-softmmu/qemu-system-x86_64 \
       -device e1000,id=e0,netdev=hn0,mac=DE:AD:BE:EF:AF:05 \
       -netdev socket,id=hn0,connect=localhost:1234
  qemu-system-x86_64: -device e1000,id=e0,netdev=hn0,mac=DE:AD:BE:EF:AF:05: Property 'e1000.netdev' can't find value 'hn0'

The old code would call net_socket_fd_init() synchronously,
while letting the connect() complete in the backgorund. The
new code moved net_socket_fd_init() so that it is only called
after connect() completes in the background.

Thus at the time we initialize the NIC frontend, the backend
does not exist.

The socket_connect() conversion as done is a bad fit for the
current code, since it did not try to change the way it deals
with async connection completion. Rather than try to fix this,
just revert the socket_connect() conversion entirely.

The code is about to be converted to use QIOChannel which
will let the problem be solved in a cleaner manner. This
revert is more suitable for stable branches in the meantime.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
