Allow creating Size instance from another Size instance

Attempt to create Size instance from another Size instance should work and
result in creation of a copy.

Related: rhbz#1077179
Make sure StorageDevice's self._size is a Size instance (#1077179)

In the size property setting the self._size attribute, we make sure nothing else
than a Size instance is assigned to self._size, but in the constructor we don't
do any such check.
All size specifications should be Size instances (#1077163)