Revert "chardev/char-socket: Fix TLS io channels sending too much data to the backend"

This commit results in unexpected termination of the TLS connection.
When 'fd_can_read' returns 0, the code goes on to pass a zero length
buffer to qio_channel_read. The TLS impl calls into gnutls_recv()
with this zero length buffer, at which point GNUTLS returns an error
GNUTLS_E_INVALID_REQUEST. This is treated as fatal by QEMU's TLS code
resulting in the connection being torn down by the chardev.

Simply skipping the qio_channel_read when the buffer length is zero
is also not satisfactory, as it results in a high CPU burn busy loop
massively slowing QEMU's functionality.

The proper solution is to avoid tcp_chr_read being called at all
unless the frontend is able to accept more data. This will be done
in a followup commit.

This reverts commit 462945cd22

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

chardev/char-socket.c

@@ -496,9 +496,9 @@ static gboolean tcp_chr_read(QIOChannel *chan, GIOCondition cond, void *opaque)
         s->max_size <= 0) {
         return TRUE;
     }
-    len = tcp_chr_read_poll(opaque);
-    if (len > sizeof(buf)) {
-        len = sizeof(buf);
+    len = sizeof(buf);
+    if (len > s->max_size) {
+        len = s->max_size;
     }
     size = tcp_chr_recv(chr, (void *)buf, len);
     if (size == 0 || (size == -1 && errno != EAGAIN)) {