virDomainDiskTranslateSourcePool: split code to setup one storage source
virDomainDiskTranslateSourcePool: split code to setup one storage source

Extract all the code setting up one storage source from the rest which
sets up the whole disk. This will allow us to prepare the whole backing
chain.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

fabianfreyer/libvirt,nertpinx/libvirt,nertpinx/libvirt,crobinso/libvirt,libvirt/libvirt,nertpinx/libvirt,nertpinx/libvirt,crobinso/libvirt,libvirt/libvirt,libvirt/libvirt,crobinso/libvirt,crobinso/libvirt,olafhering/libvirt,zippy2/libvirt,jfehlig/libvirt,jardasgit/libvirt,jardasgit/libvirt,jfehlig/libvirt,zippy2/libvirt,jardasgit/libvirt,zippy2/libvirt,olafhering/libvirt,nertpinx/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,jfehlig/libvirt,olafhering/libvirt,libvirt/libvirt,olafhering/libvirt,zippy2/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt