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

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

Use socket_*() functions from include/qemu/sockets.h instead of
listen()/bind()/connect()/parse_host_port(). socket_*() fucntions are
QAPI based and this patch  performs this api conversion since
everything will be using QAPI based sockets in the future. Also add a
helper function socket_address_to_string() in util/qemu-sockets.c
which returns the string representation of socket address. The task was
listed on http://wiki.qemu.org/BiteSizedTasks page.

Signed-off-by: Ashijeet Acharya <ashijeetacharya@gmail.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>