nbd: Switch to byte-based block access

Sector-based blk_read() should die; switch to byte-based
blk_pread() instead.

Add a constant for our magic number 512, to make it obvious
that this size will NOT change even if BDRV_SECTOR_SIZE does,
even though the two happen to be the same for now.  Split
assignments from conditionals to keep checkpatch.pl happy.

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