libct/cg/sd: check dbus.ErrClosed instead of isDbusError
libct/cg/sd: check dbus.ErrClosed instead of isDbusError

Signed-off-by: Kang Chen <8ae98a6b4bbc75e315d9597af91bae13a875554f@gmail.com>

opencontainers/runc,opencontainers/runc,opencontainers/runc,opencontainers/runc