KVM: nVMX: Update vmcs12 for SYSENTER MSRs when they're written

For L2, KVM always intercepts WRMSR to SYSENTER MSRs.  Update vmcs12 in
the WRMSR handler so that they don't need to be (re)read from vmcs02 on
every nested VM-Exit.

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