net: take ownership of fd in socket init functions

Today net/socket.c has no consistent policy for closing the socket file
descriptor when initialization fails.  This means we leak the file
descriptor in some cases or we could also try to close it twice.

Make error paths consistent by taking ownership of the file descriptor
and closing it on error.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
