blkdebug: Sanity check block layer guarantees

Commits 04ed95f4 and 1a62d0ac updated the block layer to auto-fragment
any I/O to fit within device boundaries. Additionally, when using a
minimum alignment of 4k, we want to ensure the block layer does proper
read-modify-write rather than requesting I/O on a slice of a sector.
Let's enforce that the contract is obeyed when using blkdebug.  For
now, blkdebug only allows alignment overrides, and just inherits other
limits from whatever device it is wrapping, but a future patch will
further enhance things.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20170429191419.30051-5-eblake@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
