KVM: arm64: Expose the WFXT feature to guests

Plumb in the capability, and expose WFxT to guests when available.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20220419182755.601427-8-maz@kernel.org