arm64: Add ARM64_HAS_VIRT_HOST_EXTN feature

Add a new ARM64_HAS_VIRT_HOST_EXTN features to indicate that the
CPU has the ARMv8.1 VHE capability.

This will be used to trigger kernel patching in KVM.

Acked-by: Christoffer Dall <christoffer.dall@linaro.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>