qemu-sockets: Fix compiler warning (regression for MinGW)

qemu-sockets: Fix compiler warning (regression for MinGW)

setsockopt needs a type cast for MinGW. That type cast is missing in
a recent commit which results in a compiler warning.

Like for other socket related functions which have the same problem,
we add a 'qemu_setsockopt' macro which provides that type cast where
needed and use the new macro to avoid the warning.

A 'qemu_getsockopt' is also added and can be used for future
modifications.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>