KVM: arm64: Minor cleanup of hyp variables used in host

Small cleanup moving declarations of hyp-exported variables to
kvm_host.h and using macros to avoid having to refer to them with
kvm_nvhe_sym() in host.

No functional change intended.

Signed-off-by: David Brazdil <dbrazdil@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20201208142452.87237-5-dbrazdil@google.com