KVM: VMX: Advertize RDTSC exiting to nested guests

All processors that support VMX have that feature, and guests (Xen) depend on
it.  As we already implement it, advertize it to the guest.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>