net: ensure "socket" backend uses non-blocking fds

net: ensure "socket" backend uses non-blocking fds

There are several code paths in net_init_socket() depending on how the
socket is created: file descriptor passing, UDP multicast, TCP, or UDP.
Some of these support both listen and connect.

Not all code paths set the socket to non-blocking.  This patch addresses
the file descriptor passing and UDP cases which were missing
socket_set_nonblock(fd) calls.

I considered moving socket_set_nonblock(fd) to a central location but it
turns out the code paths are different enough to require non-blocking at
different places.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>