storage: Create helper for virStorageBackendLogicalFindPoolSources
storage: Create helper for virStorageBackendLogicalFindPoolSources

Rework virStorageBackendLogicalFindPoolSources a bit to create a
helper virStorageBackendLogicalGetPoolSources that will make the
pvs call in order to generate a list of associated pv_name and vg_name's.

A future patch will make use of this for start/check processing to
ensure the storage pool source definition matches expectations.

fabianfreyer/libvirt,nertpinx/libvirt,taget/libvirt,rlaager/libvirt,jardasgit/libvirt,libvirt/libvirt,jardasgit/libvirt,zippy2/libvirt,olafhering/libvirt,datto/libvirt,taget/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,eskultety/libvirt,jardasgit/libvirt,andreabolognani/libvirt,jfehlig/libvirt,eskultety/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,libvirt/libvirt,nertpinx/libvirt,eskultety/libvirt,jfehlig/libvirt,taget/libvirt,taget/libvirt,rlaager/libvirt,VenkatDatta/libvirt,VenkatDatta/libvirt,olafhering/libvirt,crobinso/libvirt,crobinso/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,zippy2/libvirt,VenkatDatta/libvirt,nertpinx/libvirt,eskultety/libvirt,crobinso/libvirt,taget/libvirt,datto/libvirt,eskultety/libvirt,nertpinx/libvirt,jardasgit/libvirt,rlaager/libvirt,libvirt/libvirt,libvirt/libvirt,datto/libvirt,fabianfreyer/libvirt,rlaager/libvirt,datto/libvirt,nertpinx/libvirt,zippy2/libvirt,datto/libvirt,VenkatDatta/libvirt,jfehlig/libvirt,olafhering/libvirt,olafhering/libvirt,rlaager/libvirt,crobinso/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,VenkatDatta/libvirt,zippy2/libvirt