x86/vpmu: Remove core2_no_vpmu_ops

x86/vpmu: Remove core2_no_vpmu_ops

core2_no_vpmu_ops exists solely to work around the default-leaking of CPUID/MSR
values in Xen.

With CPUID handling removed from arch_vpmu_ops, the RDMSR handling is the last
remaining hook.  Since core2_no_vpmu_ops's introduction in c/s 25250ed7 "vpmu
intel: Add cpuid handling when vpmu disabled", a lot of work has been done and
the nop path in vpmu_do_msr() now suffices.

vpmu_do_msr() also falls into the nop path for un-configured or unprivileged
domains, which enables the removal the duplicate logic in priv_op_read_msr().

Finally, make all arch_vpmu_ops structures const as they are never modified,
and make them static as they are not referred to externally.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Acked-by: Jan Beulich <jbeulich@suse.com>