(MESS) corvushd: Use Corvus drive id numbers throughout. (nw)
Using the Corvus id number consistently throughout makes it easier to reason
about this code.  Previously, some functions used a 0-based drive index 
(0..14) and others used the 1-based Corvus id number (1..15).  The only
place we actually need it to be 0-based is in corvus_hdc_file(), so now we
just convert it there instead of in several places.