qcow2: Fix order in L2 table COW

When copying L2 tables (this happens only with internal snapshots), the order
wasn't completely safe, so that after a crash you could end up with a L2 table
that has too low refcount, possibly leading to corruption in the long run.

This patch puts the operations in the right order: First allocate the new
L2 table and replace the reference, and only then decrease the refcount of the
old table.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>