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
(cherry picked from commit 7f913c8254beb1d243f80754db3a1498657fba84)