Instead of declaring one BlockDriver for all host devices declared one
for each type:  a generic one for normal disk devices, a Linux floppy
driver and a CDROM driver for Linux and FreeBSD.  This gets rid of a lot
of messy ifdefs and switching based on the type in the various removal
device methods.

block.c grows a new method to find the correct host device driver based
on OS-sepcific criteria, which will later into the actual drivers in a
later patch in this series.

Signed-off-by: Christoph Hellwig <hch@lst.de>