volume: fix backup list of deleted volume
volume: fix backup list of deleted volume

When trying to list backups of a deleted volume with --volume,
openstackclient fails because the volume does not exist.
However, for listing a volume's backups, the volume itself
does not need to be present.

Fix this issue by allowing the volume of the requested backups
to not exist.

Story: 2009901
Task: 44716
Change-Id: Iea1a567151f427fa0e1ccff3af5cbeab620defcd
Signed-off-by: Jan Hartkopf <7f568ad5243604474e663e8d990521af3b476d41@inovex.de>

openstack/python-openstackclient,openstack/python-openstackclient