Adds a fallback to 'Plugin::path' in the default implementation of
'map_volume' to simplify a common case of calling 'map_volume' followed
by a defined-check and a call to path if it is not. The path is now
always returned if the plugin in question does not override
'map_volume'.
Signed-off-by: Mira Limbeck <m.limbeck@proxmox.com>
d560ec2860