Fix boot=on flag for CDROMs

This generalises commit 4304964a to cdroms too, since they have
somewhat the same logic. We just abstract the needs_boot_flag into a
separate variable, and then reuse it in the cdrom section.

Note that the logic of what 'if=' type to pass to KVM was very
convoluted, and (I think) incorrect; I went and cleaned it to be more
consistent.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>