blktap2: fix gap in tapdisk2 disk_type numbering

blktap2: fix gap in tapdisk2 disk_type numbering

Make the DISK_TYPE_* id numbering in tapdisk-disktypes.h contiguous.
Currently, id 8 is unallocated causing a null disk type entry in
tapdisk_disk_drivers array in tapdisk-disktypes.c. This causes the
function tapdisk_disktype_find() to return an error on encountering
disk types >7 (remus:, log:, etc.).

Signed-off-by: Shriram Rajagopalan <rshriram@cs.ubc.ca>
Acked-by: Daniel Stodden <daniel.stodden@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>