Wrap recv to avoid warnings

Avoid warnings like these by wrapping recv():
  CC    slirp/ip_icmp.o
/src/qemu/slirp/ip_icmp.c: In function 'icmp_receive':
/src/qemu/slirp/ip_icmp.c:418:5: error: passing argument 2 of 'recv' from incompatible pointer type [-Werror]
/usr/local/lib/gcc/i686-mingw32msvc/4.6.0/../../../../i686-mingw32msvc/include/winsock2.h:547:32: note: expected 'char *' but argument is of type 'struct icmp *'

Remove also casts used to avoid warnings.

Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

linux-user/syscall.c

@@ -2004,7 +2004,7 @@ static abi_long do_recvfrom(int fd, abi_ulong msg, size_t len, int flags,
         ret = get_errno(recvfrom(fd, host_msg, len, flags, addr, &addrlen));
     } else {
         addr = NULL; /* To keep compiler quiet.  */
-        ret = get_errno(recv(fd, host_msg, len, flags));
+        ret = get_errno(qemu_recv(fd, host_msg, len, flags));
     }
     if (!is_error(ret)) {
         if (target_addr) {