qcow2: List autoclear bit names in header

The feature table is supposed to advertise the name of all feature
bits that we support; however, we forgot to update the table for
autoclear bits.  While at it, move the table to read-only memory in
code, and tweak the qcow2 spec to name the second autoclear bit.
Update iotests that are affected by the longer header length.

Fixes: 88ddffae
Fixes: 93c24936


Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20200324174233.1622067-3-eblake@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
(cherry picked from commit bb40ebce

)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>