qcow2: Set MIN_L2_CACHE_SIZE to 2

The L2 cache must cover at least two L2 tables, because during COW two
L2 tables are accessed simultaneously.

Reported-by: Alexander Graf <agraf@suse.de>
Cc: qemu-stable <qemu-stable@nongnu.org>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Tested-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 57e21669)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>