kvm tools: add support for ARMv8 processors

This patch adds support for ARMv8 processors (more specifically,
Cortex-A57) to kvmtool. Both AArch64 and AArch32 guests are supported,
so the existing AArch32 code is slightly restructured to allow for
re-use of much of the current code.

The implementation closely follows the ARMv7 code and reuses much of the
work written there.

Tested-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>