vl: Fix error location of positional arguments

vl: Fix error location of positional arguments

We blame badness in positional arguments on the last option argument:

    $ qemu-system-x86_64 -vnc :1 bad.img
    qemu-system-x86_64: -vnc :1: Could not open 'foo': No such file or directory

I believe we've done this ever since we reported locations.  Fix it to

    qemu-system-x86_64: bad.img: Could not open 'bad.img': No such file or directory

Reported-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190318183312.4684-1-armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>