virsh: kill over-engineered asprintf failure recovery
    
    I noticed this while shortening switch statements via VIR_ENUM.
    Basically, the only ways virAsprintf can fail are if we pass a
    bogus format string (but we're not THAT bad) or if we run out
    of memory (but it already warns on our behalf in that case).
    Throw away the cruft that tries too hard to diagnose a printf
    failure.
    
    * tools/virsh-volume.c (cmdVolList): Simplify.
    * tools/virsh-pool.c (cmdPoolList): Likewise.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>