virsh: support VISUAL, and allow metacharacters in EDITOR

virsh: support VISUAL, and allow metacharacters in EDITOR

Common Unix practice is to prefer VISUAL over EDITOR, particularly if
the editor of choice spawns a new window.  Thus, it is also common to
see settings like EDITOR='emacs -nw', with the expectation that the
shell will parse this as an argument to 'emacs' and not try to invoke
a file containing a space.

If a user puts junk in EDITOR, they deserve what they get (much more
than virsh will misbehave); furthermore, sudo scrubs EDITOR by
default.  So the blind use of metacharacters in EDITOR should not be
considered too much of a security issue.

* tools/virsh.c (editFile): Prefer VISUAL over EDITOR.  Don't
reject shell metacharacters in EDITOR.
* tools/virsh.pod (edit, net-edit, ENVIRONMENT): Document VISUAL.
Fixes https://bugzilla.redhat.com/show_bug.cgi?id=487738.