libvhost-user: add vmsg_set_reply_u64() helper

libvhost-user: add vmsg_set_reply_u64() helper

The VhostUserMsg request is reused as the reply by message processing
functions.  This is risky since request fields may corrupt the reply if
the vhost-user message handler function forgets to re-initialize them.

Changing this practice would be very invasive but we can introduce a
helper function to make u64 payload replies safe.  This also eliminates
code duplication in message processing functions.

Signed-off-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Marc-André Lureau <[email protected]>
Message-Id: <20190626074815[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>