First step cleaning up the drives handling.  This one does nothing but
removing drives_table[], still it became seriously big.

drive_get_index() is gone and is replaced by drives_get() which hands
out DriveInfo pointers instead of a table index.  This needs adaption in
*tons* of places all over.

The drives are now maintained as linked list.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>