Fix invalid vdagent buffer access

The caller use the "size" argument in different ways. Either the size of the data to convert, or the end boundary to be deduced by offset.

Fix it so the the "size" argument means the amount in bytes of data to convert, that seems simpler and saner. (yay C)

Fixes: spice#53 (closed)

Signed-off-by: Marc-André Lureau marcandre.lureau@redhat.com