qcow2: Rework qcow2_snapshot_create error handling

Increase refcounts only after allocating a new L1 table has succeeded in
order to make leaks less likely. If writing the snapshot table fails,
revert in-memory state to be consistent with that on disk.

While at it, make it return the real error codes instead of -1.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
