Fix incorrect volume group naming when vg_name is not specified

[presztak]

In the context of clustering, when vg_name is not specified, the name of the storage pool that the user wants to create is used as the volume group name. This approach is incorrect if the source is a volume group name, as it leads to a mismatch between the source (volume group name) and the storage pool name, causing issues.

To address this, we need to detect such cases. In other scenarios, such as when a physical device is specified as the source, the storage pool name provided by the user should be used as the volume group name.

To solve this issue, a helper method was implemented to determine the source type based on the value of config["source"]. This method works as follows:

If the source is empty, a loop file, or a physical device, the storage pool name is used as the volume group name.
If the source is a volume group name, then that name is used as the volume group name.
This helper method ensures consistent naming logic and is now used in both the init() and Create() methods.

Fixes: #1650