null: Switch to byte-based read/write

null: Switch to byte-based read/write

We are gradually moving away from sector-based interfaces, towards
byte-based.  Make the change for the last few sector-based callbacks
in the null-co and null-aio drivers.

Note that since the null driver does nothing on writes, it trivially
supports the BDRV_REQ_FUA flag (all writes have already landed to
the same bit-bucket without needing an extra flush call).  Also, since
the null driver does just as well with byte-based requests, we can
now avoid cycles wasted on read-modify-write by taking advantage of
the block layer now defaulting the alignment to 1 instead of 512.

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