nixos/tests/blivet: Fix btrfs-related tests

The loopback-based tests use a storage size of 102400 blocks (one block
is 1024 bytes), which doesn't seem to fit for btrfs volumes in recent
btrfs versions. I'm setting this to 409600 (400 MB) now so that it
should be enough for later versions in case they need even more space
for subvolumes.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>