only 256kb data can be sent via viosock from Linux host to Windows guest

[xuehuihui]

Describe the bug
I try to send a file from linux to windows guest, but only 256kb length can be successfully sent every time.
send api will return '-1', errno is is set to EAGAIN.

Additional context
I found out it was because the guest did not send CREDIT_UPDATE packet to the host. so cause the buffer to fill up.

resolve
Rx.c:1437 pCurrentCb->BytesToRead = 0;
it will cause fwd_cnt to always be 0. When I comment this line of code, everything works fine. So I wonder what is the purpose here

[xuehuihui]

@irudakov77

[YanVugenfirer]

@MartinDrab FYI