ide: Check validity of logical block size

Our IDE emulation can't handle logical block sizes other than 512. Check
for it.

The original assumption was that other values would silently be ignored
(which is bad enough), but it's not quite true: The physical block size
is exposed in IDENTIFY DEVICE as a multiple of the logical block size.
Setting a logical block size therefore also corrupts the physical block
size (4096/4096 doesn't silently downgrade to 4096/512, but 512/512).

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
