storage: FS backend adjust error message on error path
storage: FS backend adjust error message on error path

https://bugzilla.redhat.com/show_bug.cgi?id=1181087

Currently the assumption on the error message is that there are
no source device paths defined when the number of devices check
fails, but in reality the XML could have had none or it could have
had more than the value supported. Adjust the error message accordingly
to make it clearer what the error really is.

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