Make _minInstanceSize, a source of minSize() value, always a Size.
Make _minInstanceSize, a source of minSize() value, always a Size.

This will make minSize be a Size more often and None less often, probably
never.

Remove class level variable _minInstanceSize in TmpFS. _minInstanceSize is
always set to Size(0) in FS.__init__(), and is supposed to be an instance
variable.

Signed-off-by: mulhern <7b51bcf507bcd7afb72bf8663752c0ddbeb517f6@redhat.com>

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