[ARM] Add feature and cpuid for generic ARMv8 VCPU

Upcoming arm64 port would require us to distinguish between
ARMv7 VCPUs and ARMv8 VCPUs hence adding ARM_FEATURE_V8 and
ARM_CPUID_ARMV8 for a generic ARMv8 VCPU in-advance.

Signed-off-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Sukanto Ghosh <sukantoghosh@gmail.com>