Add more sanity checking for rootfs size (#468867)

[Jeremy Katz <katzj redhat com>]

The following two patches add the ability for backends to define a way to 
tell how much space they need for a specific partition, an implementation
for / on the livecd and the checking of it so that we can make it an error
when people do partitioning.

It's possible that you can't know at the stage when it's asked, at which
point we just return 0 and aren't worse off than we were before.