qemu-img: fix switch indentation in img_amend()

QEMU coding style indents 'case' to the same level as the 'switch'
statement:

  switch (foo) {
  case 1:

Fix this coding style violation so checkpatch.pl doesn't complain about
the next patch.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20170317104541.28979-3-stefanha@redhat.com
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Max Reitz <mreitz@redhat.com>