KVM: nVMX: Lift sync_vmcs12() out of prepare_vmcs12()

... to make it more obvious that sync_vmcs12() is invoked on all nested
VM-Exits, e.g. hiding sync_vmcs12() in prepare_vmcs12() makes it appear
that guest state is NOT propagated to vmcs12 for a normal VM-Exit.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>