tools/blktap2: Fix array initialisers for tapdisk_disk_{types,drivers}[]

tools/blktap2: Fix array initialisers for tapdisk_disk_{types,drivers}[]

Clang points out:

  tapdisk-disktype.c:117:2: error: initializer overrides prior initialization
  of this subobject [-Werror,-Winitializer-overrides]
          0,
          ^
  tapdisk-disktype.c:115:23: note: previous initialization is here
          [DISK_TYPE_VINDEX]      = &vhd_index_disk,
                                    ^~~~~~~~~~~~~~~

Mixing different initialiser styles should be avoided; The actual behaviour is
different to the expected behaviour.  This specific example has been broken
since its introduction in c/s 7b4dea554 "blktap2: Fix tapdisk disktype issues"
in 2010, and is caused by the '#if 0' block removing &tapdisk_{sync,vmdk}.

First of all, remove what were intended to be trailing NULL entries in
tapdisk_disk_{types,drivers}[], making consistent use of Designated
Initialisers for the initialisation.

This requires changing the loop in tapdisk_disktype_find() to be based on the
number of elements in tapdisk_disk_types[], rather than looking for the first
NULL.  This fixes a latent bug, as the use of Designated Initializers causes
to intermediate zero entries if not all indices are explicitly specified.

There is a second latent bug where tapdisk_disktype_find() assumes that
tapdisk_disk_drivers[] has at least as many entries as tapdisk_disk_types[].
This is not the case and tapdisk_disk_drivers[] had one entry fewer than
tapdisk_disk_types[], but the NULL loop bound prevented an out-of-bounds read
of tapdisk_disk_drivers[].  Fix the issue by explicitly declaring
tapdisk_disk_drivers[] to have the same number of entries as
tapdisk_disk_types[].

Finally, this leads to a linker error.  It turns out that tapdisk_vhd_index
doesn't exist, and I can't find any evidence in the source history to suggest
that it ever did.  I can only presume that it would have been #if 0'd out like
tapdisk_sync and tapdisk_vmdk had it not been for this bug preventing a build
failure.  Drop all three.

No functional change, but only because of the specific layout of
tapdisk_disk_types[].

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Doug Goldstein <cardoe@cardoe.com>