net/socket: Fix compiler warning (regression for MinGW)

net/socket: Fix compiler warning (regression for MinGW)

Commit 213fd5087e2e4e2da10ad266df0ba950cf7618bf removed a type cast
which is needed for MinGW:

net/socket.c:136: warning:
 pointer targets in passing argument 2 of ‘sendto’ differ in signedness
/usr/lib/gcc/amd64-mingw32msvc/4.4.4/../../../../amd64-mingw32msvc/include/winsock2.h:1313: note:
 expected ‘const char *’ but argument is of type ‘const uint8_t *’

Add a 'qemu_sendto' macro which provides that type cast where needed
and use the new macro instead of 'sendto'.

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