virsh: Create macros for common "vol" options
    
    Rather than continually cut-n-paste the strings into each command,
    create common macros to be used generically.  Replace the more commonly
    used "vol" option with a macro. This also adjusts 2 commands that
    didn't have the correct helpstr - 'vol-create-from' and 'vol-clone'.
    Both are described in the man page as taking vol, path, or key and
    the code uses the virshCommandOptVol instead of virshCommandOptVolBy.
    
    Signed-off-by: John Ferlan <jferlan@redhat.com>