x86/vPMU: convert vendor hook invocations to altcall

x86/vPMU: convert vendor hook invocations to altcall

At least some vPMU functions will be invoked (and hence can further be
speculated into) even in the vPMU-disabled case. Convert vpmu_ops to
the standard single-instance model being a prerequisite to engaging the
alternative_call() machinery, and convert all respective calls. Note
that this requires vpmu_init() to become a pre-SMP initcall.

This change then also helps performance.

To replace a few vpmu->arch_vpmu_ops NULL checks, introduce a new
VPMU_INITIALIZED state, such that in the absence of any other suitable
vmpu_is_set() checks this state can be checked for.

While adding the inclusion of xen/err.h, also prune other xen/*.h
inclusions.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>