Parse volname where needed

Parse volname where needed

The common ZFSPlugin was missing volume name parsing
in a few places. This was not a problem for standard
volumes, but broke functionnalities (like resize,
snapshot, rollback) with linked clones as the name of
the zvol must be extracted from the entry in the config
(remove base-X-disk-Y prefix)

Signed-off-by: Daniel Berteaud <daniel@firewall-services.com>