virsh: add 'exit' as an alias for 'quit'

virsh: add 'exit' as an alias for 'quit'

Call me lazy: some shells use exit (e.g. sh), others use quit (e.g. ftp),
but I never remember which.  So it's faster to write a patch to make
virsh take both than it is to take a 50-50 guess, and get it wrong
in half of my attempts.

* tools/virsh.c (commands): Add 'exit'.
* tools/virsh.pod: Document it.