conf: Introduce optional startupPolicy attribute for cdrom and floppy

conf: Introduce optional startupPolicy attribute for cdrom and floppy

This attribute says what to do with cdrom (or floppy) if
the source is missing. It accepts:
- mandatory - fail if missing for any reason (the default)
- requisite - fail if missing on boot up, drop if missing on
              migrate/restore/revert
- optional  - drop if missing at any start attempt.

However, this patch introduces only XML part of this new
functionality.