net: asynchronous send/receive infrastructure for net/socket.c

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>
(cherry picked from commit 863f678fba4191f3b695620f41056cb7c124425d)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>