cmdlib: use BTRFS for the cache2.qcow2

We switched to ext4 to get away from using reflinks (see [1])
but the problem was actually specific to XFS. It's now been
fixed upstream in [2], but we need to wait for it to get back
backported to stable branches. Let's switch to BTRFS for now
so we can still get the space savings of reflinks, while avoiding
the reflink bug in XFS.

[1] coreos#3728
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=4b2f459d86252619448455013f581836c8b1b7da

src/cmdlib.sh

@@ -608,7 +608,7 @@ runvm_with_cache_snapshot() {
         (
          # shellcheck source=src/libguestfish.sh
          source /usr/lib/coreos-assembler/libguestfish.sh
-         virt-format --filesystem=ext4 --label=cosa-cache -a cache2.qcow2.tmp)
+         virt-format --filesystem=btrfs --label=cosa-cache -a cache2.qcow2.tmp)
         mv -T cache2.qcow2.tmp "${workdir}"/cache/cache2.qcow2
     fi
     # And remove the old one