qcow: Fix qcow1 exit fault

Even though qcow1 doesn't use the refcount table, the cleanup path still
attempts to iterate over its LRU list. Initialize the list to avoid a
segfault on exit.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>