base: extend vm_session API with native vcpu cap

To enable the interaction of a VMM with the kernel directly,
a hidden RPC gets introduced. It allows a kernel-specific
base-library implementation of the Vm_session::Client to request
a kernel-specific capability to address a VCPU, e.g., to
run/stop it.

Ref #3926

repos/base-sel4/src/core/include/vm_session_component.h

@@ -121,6 +121,8 @@ class Genode::Vm_session_component
 		void attach_pic(addr_t) override {}
 		void detach(addr_t, size_t) override;
 		Vcpu_id _create_vcpu(Thread_capability);
+		Capability<Native_vcpu> _native_vcpu(Vcpu_id) {
+			return Capability<Native_vcpu>(); }
 };
 
 #endif /* _CORE__VM_SESSION_COMPONENT_H_ */