src: cap the data size in stream I/O functions

src: cap the data size in stream I/O functions

The main stream I/O functions have a design flaw in that they accept
'size_t' as the input data length, while intending to return the
amount actually processed in an 'int'.

Fortunately all functions explicitly document that less data may be
processed than requested, and with the remote driver data cap we will
never get anywhere near exceeding an 'int' even on 32-bit.

For sanity, however, lets explicitly cap the data size in the public
API to fix the design flaw.

Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>