KVM: selftests: state_test: test bare VMXON migration

Split prepare_for_vmx_operation() into prepare_for_vmx_operation() and
load_vmcs() so we can inject GUEST_SYNC() in between.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>