`Vm_session_component::create_vcpu()` is present across all supported
kernels, yet until now it was not part of the `Vm_session` interface.
Add the method to the `Vm_session` interface. This unifies calls in the
base library and is the basis to remove the need for a common base class
for separate `Vm_session` implementations for SVM and VMX on x86_64.
Issue #5221
5e3a898109