luks: Allow share-rw=on

luks: Allow share-rw=on

Format drivers such as qcow2 don't allow sharing the same image between
two QEMU instances in order to prevent image corruptions, because of
metadata cache. LUKS driver don't modify metadata except for when
creating image, so it is safe to relax the permission. This makes
share-rw=on property work on virtual devices.

Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>