net: asynchronous send/receive infrastructure for net/socket.c The net/socket.c net client is not truly asynchronous. This patch borrows the qemu_set_fd_handler2() code from net/tap.c as the basis for proper asynchronous send/receive. Only read packets from the socket when the peer is able to receive. This avoids needless queuing. Later patches implement asynchronous send. Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>