virsh: Create macro for common "config" option
    
    Rather than continually cut-n-paste the strings into each command,
    create a common macro to be used generically. The macro will take a
    single argument _helpstr which for many options in virsh-domain.c
    is simply "affect next boot". So, create a second macro within that
    file in order to define the more common use as a revector to the
    common macro with the common _helpstr.
    
    Signed-off-by: John Ferlan <jferlan@redhat.com>