qemu-socket: set passed fd non-blocking in socket_connect()

socket_connect() sets non-blocking on TCP or UNIX domain sockets if a
callback function is passed.  Do the same for file descriptor passing,
otherwise we could unexpectedly be using a blocking file descriptor.

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