storage: Create helper to generate FS pool source value
storage: Create helper to generate FS pool source value

Refactor the code that builds the pool source string during the FS
storage pool mount to be a separate helper.

A future patch will use the helper in order to validate the mounted
FS matches the pool's expectation during poolCheck processing

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