mirror: Switch MirrorBlockJob to byte-based

We are gradually converting to byte-based interfaces, as they are
easier to reason about than sector-based.  Continue by converting an
internal structure (no semantic change), and all references to the
buffer size.

Add an assertion that our use of s->granularity >> BDRV_SECTOR_BITS
(necessary for interaction with sector-based dirty bitmaps, until
a later patch converts those to be byte-based) does not suffer from
truncation problems.

[checkpatch has a false positive on use of MIN() in this patch]

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
