virsh: Create macro for common "current" option

virsh: Create macro for common "current" 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 current domain". 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>