vdagent: Fix udscs_read_callback memory ownership bug
Previous commit changed ownership rules for the data passed to the
udscs_read_callback, but forgot to update one of the user.
Both spice-vdagent and spice-vdagentd use an udscs_read_callback, either
from udscs_connect() or udscs_create_server().
The previous commit only updated users of udscs_create_server(). This
caused memory corruption in spice-vdagent which uses udscs_connect().
This fixes https://bugs.freedesktop.org/show_bug.cgi?id=98830