virsh: Parse # comments in batch mode

virsh: Parse # comments in batch mode

Continuing from what I did in commit 4817dec0, now I want to write a
sequence that is self-documenting.  So I need comments :)

Now I can do something like:

$ virsh -c test:///default '
  # setup
  snapshot-create-as test s1
  snapshot-create-as test s2
  # check
  snapshot-list test --name
'

Note that this does NOT accept comments in argv mode, another patch
will tackle that.

(If I'm not careful, I might turn virsh into a full-fledged 'sh'
replacement? Here's hoping I don't go that far...)

Signed-off-by: Eric Blake <eblake@redhat.com>
Acked-by: Michal Privoznik <mprivozn@redhat.com>