Mer: Fix start/stop VM icon handling

Do not use action.setIcon(icon.pixmap(...)) to switch between START and
STOP variant of the icon. Use two QIcon instances instead to prevent
scaling issues on (non)retina displays[1].

Note that single QIcon together with QAction::checked property could not
be used - unfortunately the buttons do not obey this property.

[1] http://blog.qt.digia.com/blog/2013/04/25/retina-display-support-for-mac-os-ios-and-x11/

Signed-off-by: Martin Kampas <martin.kampas@jolla.com>