x86: softtsc for PV domains

x86: softtsc for PV domains

Implement softtsc (TSC emulation) for userland code in PV domains.  It
currently is tied to the existing "softtsc" Xen boot option (which
does the same thing but for HVM domains).  Later it should be tied to
a vm.cfg option, but this is sufficient for now to obtain performance
degradation data for PV environments that heavily utilize rdtsc.  To
record emulation frequency, use debug-key "s".

Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>