KVM: selftests: Sync stage before VM is freed in hypercalls test

Sync the next stage using the VM before said VM is potentially freed by
the TEST_STAGE_HVC_IFACE_FEAT_DISABLED stage.

Opportunistically take a double pointer in anticipation of also having to
set the new vCPU pointer once the test stops hardcoding '0' everywhere.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>