qemu-img: Error out for excess arguments

Don't silently ignore excess arguments at the end of the command line,
but error out instead. This can catch typos like 'resize test.img + 1G',
which doesn't increase the image size by 1G as intended, but truncates
the image to 1G. Even for less dangerous commands, the old behaviour is
confusing.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>