KVM: arm64: Move VHE direct sysreg accessors into kvm_host.h

As we are about to need to access system registers from the HYP
code based on their internal encoding, move the direct sysreg
accessors to a common include file, with a VHE-specific guard.

No functionnal change.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>