alloca one extra byte sockets

target_to_host_sockaddr() may increase the lenth with 1 byte
for AF_UNIX sockets so allocate 1 extra byte.

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>