More generic boot devices specification, allowing more devices to be specified
and avoiding per-target hardcoded limitations.
The machine implementations can then check if the given devices match the
actual hardware implementation and firmware API.


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@3577 c046a42c-6fe2-441c-8c8c-71466251a162
