qemu-img: snapshot: make -l (list) the default, simplify option handling

qemu-img: snapshot: make -l (list) the default, simplify option handling

When no -l/-a/-c/-d specified, assume -l (list).

Use the same values for SNAPSHOT_LIST/etc constants as the
option chars (lacd), this makes it possible to simplify
option handling a lot, combining cases for 4 options into
one.

Also remove bdrv_oflags handling (only list can use RO mode).

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <20250531171609.197078-17-mjt@tls.msk.ru>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>