Eliminate blocking D-Bus calls in QOfonoModem

Now you have one more place to discuss deleteLater()... one more place to care about things you should not care, additionally to 50 more places in libqofono only. At least, there were several PRs to fix leaks because of missing deleteLater()...
Ok, this is traditional way... (https://github.com/nemomobile/libqofono/pull/56#issuecomment-63798194)