[roll] Roll fuchsia [fvm] Support the backup superblock in blobfs

fxrev.dev/459062 added a backup superblock to blobfs which is
only present in FVM images.

That CL made it so that newly created blobfs images would have this
backup superblock set up properly, but it did not do the same treatment
for the code in fvm/host which expands a blobfs image into an FVM image.
This meant that such images would not have a backup superblock
populated, which fails fsck. (Normal usage still works, since blobfs
does not consult the backup superblock in normal usage.)

Fixed this, and added two new tests:
- A test in fvm_sparse_image_reader_test which exercises loading a
  sparse FVM image and fscking its data/blobfs partitions
- A host test for fvm/host which performs a very basic smoke-test
  of generating a trivial image and fsck-ing said image

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/467883
Original-Revision: 7a89c0db533318e21340a129ea2a7395ec7a78d0
GitOrigin-RevId: e3d9575cc6a8fd8118e91b487e4d3ad28630137c
Change-Id: I0768dda8ad6751983e9f9d5a6bb1a54989385c99