virsh: don't print --(null) in vol-name and vol-pool
virsh: don't print --(null) in vol-name and vol-pool

Don't print the pool option name if it's null.

Before:
virsh # vol-name vol
error: failed to get vol 'vol', specifying --(null) might help
error: Storage volume not found: no storage vol with matching path vol

After:

virsh # vol-name vol
error: failed to get vol 'vol'
error: Storage volume not found: no storage vol with matching path vol

Bug: https://bugzilla.redhat.com/show_bug.cgi?id=924571

rlaager/libvirt,zhlcindy/libvirt-1.1.4-maintain,olafhering/libvirt,rlaager/libvirt,zhlcindy/libvirt-1.1.4-maintain,crobinso/libvirt,crobinso/libvirt,fabianfreyer/libvirt,rlaager/libvirt,zippy2/libvirt,jardasgit/libvirt,eskultety/libvirt,cbosdo/libvirt,VenkatDatta/libvirt,crobinso/libvirt,rlaager/libvirt,zippy2/libvirt,elmarco/libvirt,taget/libvirt,zhlcindy/libvirt-1.1.4-maintain,fabianfreyer/libvirt,cbosdo/libvirt,libvirt/libvirt,elmarco/libvirt,VenkatDatta/libvirt,zhlcindy/libvirt-1.1.4-maintain,eskultety/libvirt,fabianfreyer/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,jfehlig/libvirt,libvirt/libvirt,nertpinx/libvirt,eskultety/libvirt,cbosdo/libvirt,zippy2/libvirt,eskultety/libvirt,datto/libvirt,agx/libvirt,libvirt/libvirt,nertpinx/libvirt,rlaager/libvirt,jfehlig/libvirt,shugaoye/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,zippy2/libvirt,cbosdo/libvirt,elmarco/libvirt,taget/libvirt,jfehlig/libvirt,agx/libvirt,agx/libvirt,zhlcindy/libvirt-1.1.4-maintain,jardasgit/libvirt,nertpinx/libvirt,eskultety/libvirt,datto/libvirt,VenkatDatta/libvirt,cbosdo/libvirt,libvirt/libvirt,taget/libvirt,crobinso/libvirt,olafhering/libvirt,jardasgit/libvirt,datto/libvirt,elmarco/libvirt,agx/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,VenkatDatta/libvirt,datto/libvirt,taget/libvirt,shugaoye/libvirt,nertpinx/libvirt,datto/libvirt,shugaoye/libvirt,elmarco/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,nertpinx/libvirt,taget/libvirt,shugaoye/libvirt,agx/libvirt,shugaoye/libvirt,andreabolognani/libvirt,olafhering/libvirt,olafhering/libvirt