Allow creating Size instance from another Size instance
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

vpodzime/blivet,rvykydal/blivet,jkonecny12/blivet,AdamWill/blivet,jkonecny12/blivet,rhinstaller/blivet,vpodzime/blivet,rhinstaller/blivet,dwlehman/blivet,vojtechtrefny/blivet,vojtechtrefny/blivet,rvykydal/blivet,AdamWill/blivet,dwlehman/blivet