Allow KVM to boot from HTTP

New versions of KVM support booting from HTTP-hosted ISO
images, via libcurl. This patch adds a proper check to
allow defining either a sane, absolute path or an HTTP
URL as an iso image path.

Remove "format=raw" from the cdrom device options when iso_image
starts with "(https?|ftp)://", to allow latest KVM versions (qemu-kvm) to
boot from HTTP-hosted iso images. Added FILE_OR_URL check to allow
booting from HTTP (kvm).

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>