VMX: x86: handle host TSC calibration failure

If the host TSC calibration fails, tsc_khz is zero (see tsc_init.c).
Handle such case properly in KVM (instead of dividing by zero).

https://bugzilla.redhat.com/show_bug.cgi?id=859282



Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>