For arbitrary shapes you can use the boolean modifier, if you set the threshold to 0.000000 it can even generate nice results in geometry with overlapping faces (sometimes). A further boolean operation with an XY plane would be needed to get rounded corners. This does require that you start from a Mesh rather than an imported SVG/Curve. That will give varying degrees of success.
Bevel... just Bevel
Make a Cuboid, select all faces and hit Ctrl+B, then use mouse wheel to adjust segment count and "mouse distance travelled" to set the radius. If you want precise input of radius and segment count look at the properties display at the bottom of the 3dview - you can type a number directly to get the radius (offset) and press S to start typing the number of segments.
Alternatively for a rounded cuboid, you could use a dedicated addon for that. Rounded Cube by phymec. That way you can give corners radius control, parametrically for the lifetime of the operator.
Sverchok also has a rounded cube node (uses the same code as the Rounded Cube add-on)