`vmctl exec` just wraps `ssh`, so the command arguments are interpreted
as such. That means, the arguments are joined by space and passed to
`sh -c "$*"` on the remote side.

That means, any argument passed there really need to be a shell script
(after joining with spaces). That may not be obvious to the caller and
it can also be cumbersome to get quoting/escaping right.

There is no change in behavior here, just add some comments to make the
clearer.

Signed-off-by: Thomas Haller <thaller@redhat.com>