KVM: PPC: Book3S HV: XIVE: Add a 'flags' field

Use it to hold platform specific features. P9 DD2 introduced
single-escalation support. P10 will add others.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210720134209.256133-2-clg@kaod.org