x86/VPMU: interface for setting PMU mode and flags

x86/VPMU: interface for setting PMU mode and flags

Add runtime interface for setting PMU mode and flags. Three main modes are
provided:
* XENPMU_MODE_OFF:  PMU is not virtualized
* XENPMU_MODE_SELF: Guests can access PMU MSRs and receive PMU interrupts.
* XENPMU_MODE_HV: Same as XENPMU_MODE_SELF for non-proviledged guests, dom0
  can profile itself and the hypervisor.

Note that PMU modes are different from what can be provided at Xen's boot line
with 'vpmu' argument. An 'off' (or '0') value is equivalent to XENPMU_MODE_OFF.
Any other value, on the other hand, will cause VPMU mode to be set to
XENPMU_MODE_SELF during boot.

For feature flags only Intel's BTS is currently supported.

Mode and flags are set via HYPERVISOR_xenpmu_op hypercall.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Acked-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Dietmar Hahn <dietmar.hahn@ts.fujitsu.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>