qemu-io: Switch to byte-based block access

qemu-io: Switch to byte-based block access

qemu-io is the last user of several sector-based interfaces.
This patch upgrades to the new interfaces under the hood,
then deletes the resulting dead code.  Note that for maximum
back-compat, while the -p option is no longer required to get
blk_pread(), it is still needed to allow for unaligned access;
this is because qemu-iotest 23 relies on qemu-io rejecting
unaligned accesses without -p.  A later patch may clean up the
interface to be more user-friendly, but it's better to separate
what's done under the hood from what the user sees.

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