xen/PMU: PMU emulation code

xen/PMU: PMU emulation code

Add PMU emulation code that runs when we are processing a PMU interrupt.
This code will allow us not to trap to hypervisor on each MSR/LVTPC access
(of which there may be quite a few in the handler).

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
(cherry picked from commit bf6dfb154d935725c9a2005033ca33017b9df439)
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>