qcow2: Allow -o compat=v3 during qemu-img amend

Commit b76b4f60 allowed '-o compat=v3' as an alias for the
less-appealing '-o compat=1.1' for 'qemu-img create' since we want to
use the QMP form as much as possible, but forgot to do likewise for
qemu-img amend.  Also, it doesn't help that '-o help' doesn't list our
new preferred spellings.

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