util: storage: Fix virStorageSourceGetActualType if volume was not translated

util: storage: Fix virStorageSourceGetActualType if volume was not translated

virStorageSourceGetActualType would return VIR_STORAGE_TYPE_NONE in case
when a virStorageSource of (top level) type VIR_STORAGE_TYPE_VOLUME was
not prepared to use by the vm by calling
virDomainDiskTranslateSourcePool.

Fix this issue by returning VIR_STORAGE_TYPE_VOLUME in case when the
volume was not translated yet.

Additionally also add documentation for the function describing the
quirk.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>