This option allows to set the extension of the ISO file after download.
Defaults to "iso". It makes sense for building Mac OS X guests, where the
bootable image is actually a DMG, not an ISO.
In particular, it is important for "parallels-iso" builder to set the right extension.
When dealing with windows the file url format is file:///c:/
On windows a lot of git clients will convert LF to CRLF. This would be a problem where file contents are compared exactly
The ISO builders (parallels, qemu, virtualbox, and vmware) had too
much common code which needed to be maintained separately. This change
moves that code to a common ISO configuration.