slirp: fix guest network access with darwin host

slirp: fix guest network access with darwin host

On Darwin, connect, sendto and friends want the exact size of the sockaddr,
not more (and in particular, not sizeof(struct sockaddr_storaget))

This commit adds the sockaddr_size helper to be used when passing a sockaddr
size to such function, and makes use of it int sendto and connect calls.

Signed-off-by: Samuel Thibault <[email protected]>
Reviewed-by: John Arbuckle <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>