qemu-img resize: Fix option parsing

For shrinking images, you're supposed to use a negative size. However, the
leading minus makes getopt think that it's an option and so you get the help
text if you don't use -- like in 'qemu-img resize test.img -- -1G'.

This patch handles the size first and removes it from the argument list so that
getopt won't even try to interpret it and you don't need -- any more.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
