conf: storage_source: Introduce type for storing FDs associated for storage

conf: storage_source: Introduce type for storing FDs associated for storage

For FD-passing of disk sources we'll need to keep the FDs around.
Introduce a data type helper based on a g_object so that we get
reference counting.

One instance will (due to security labelling) will need to be part of
the virStorageSource struct thus it's declared in the storage_source_conf
module.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>