x86,kvm,vmx: Don't trap writes to CR4.TSD

CR4.TSD is guest-owned; don't trap writes to it in VMX guests.  This
avoids a VM exit on context switches into or out of a PR_TSC_SIGSEGV
task.

I think that this fixes an unintentional side-effect of:
    4c38609a

 KVM: VMX: Make guest cr4 mask more conservative

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>