conf: define a new "maximum" CPU mode

conf: define a new "maximum" CPU mode

For hardware virtualization this is functionally identical to the
existing host-passthrough mode so the same caveats apply.

For emulated guest this exposes the maximum featureset supported by
the emulator. Note that despite being emulated this is not guaranteed
to be migration safe, especially if different emulator software versions
are used on each host.

Reviewed-by: Pavel Hrdina <phrdina@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>