x86/hvm: Don't expose CPUID time leaf when not using PVRDTSCP

x86/hvm: Don't expose CPUID time leaf when not using PVRDTSCP

Some versions of Oracle's Solaris PV drivers make a check that the
maximal Xen hypervisor CPUID leaf is <= base leaf + 2 and refuse to
work if this is not the case.  The addition of the time leaf makes the
maximal leaf == base leaf + 3 so this patch introduces a workaround
that obscures the time leaf unless PVRDTSCP is in operation.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>