sysroot: Cache deployment device/inode

sysroot: Cache deployment device/inode

This is a lot cleaner. In particular, it's prep
for having soft reboot finalize at shutdown time, because
we weren't correctly handling the staged deployment in
that case.

To implement this we need to basically stop using
ostree_deployment_new() in most places, and instead ensure that all
callers use this path which cache the device/inode.

Signed-off-by: Colin Walters <walters@verbum.org>