This adds a check for the root device having filesystem type btrfs,
and when it is assume there is a subvolume called "root". This fixes
extract-image when using Fedora-Cloud-Base btrfs images.
This should be sufficient until there is another btrfs base image with
a different subvolume layout.
Change-Id: Ib18979090585ba92566e523951b521b9d902fcb7