KVM: arm64: Move the host hypercall interface to its own header

Move the kvm_call_hyp() dispatch macros and pkvm_handle_t out of
kvm_host.h into a new kvm_hcall.h, giving the host<->hyp hypercall
interface a single home that subsequent patches build on to restore
type-checking across the boundary. The only adjustment to the moved
code is the checkpatch-mandated space in "while (0)".

No functional change intended.

Reviewed-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>