sysroot: Clean up API

Now that we have a real GObject for the sysroot, we have a convenient
place to keep track of 4 pieces of state:

* The current deployment list
* The current bootversion
* The current subbootversion
* The current booted deployment (if any)

Avoid requiring callers to pass all of this around and load it
piecemeal; instead the new thing is ostree_sysroot_load().