qcow2: Use consistent switch indentation

Fix a couple of inconsistent indentations, before an upcoming
patch further tweaks the switch statements.
(best viewed with 'git diff -b').

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20170507000552.20847-3-eblake@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>