block: Add '--version' option to qemu-img

This allows qemu-img to print out version information, without
needing to print the long help wall of text.

While there, perform some minor whitespace cleanup, and remove the
unused option_index variable in the call to getopt_long().

Reported-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>