Add support for building QEMU images from pre-existing image files

If the `disk_image` parameter is set to true treat the ISO location
as the source of the disk image, copy it to the destination,
resize and boot it.
Update QEMU builder documentation with `disk_image` flag
Merge pull request #1342 from iwebhosting/qemu-image-support

Add support for building QEMU images from pre-existing image files