Revert "Make default invocation of block drivers safer (v3)"

This reverts commit 79368c81.

Conflicts:

	block.c

I haven't been able to come up with a solution yet for the corruption caused by
unaligned requests from the IDE disk so revert until a solution can be written.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
(cherry picked from commit 8b33d9ee)