virsh: Convert '<cmd> --help' to 'help <cmd>'

virsh: Convert '<cmd> --help' to 'help <cmd>'

Often times I find myself halfway through typing a long command when
I want to see 'help' output. I instinctively append '--help' to the
command I'm typing, only to get an error:

    $ virsh vol-create-as foo --help
    error: command 'vol-create-as' doesn't support option --help

This patch makes --help work in a pretty hacky way. One missing piece
here is that --help isn't listed as an option in the actual 'help <cmd>'
output, but maybe this can be a starting point for someone.