kvm: Implement kvm_irqchip_in_kernel like kvm_enabled

kvm: Implement kvm_irqchip_in_kernel like kvm_enabled

To both avoid that kvm_irqchip_in_kernel always has to be paired with
kvm_enabled and that the former ends up in a function call, implement it
like the latter. This means keeping the state in a global variable and
defining kvm_irqchip_in_kernel as a preprocessor macro.

Signed-off-by: Jan Kiszka <[email protected]>
Signed-off-by: Marcelo Tosatti <[email protected]>