xen/sched: Remove d->is_pinned

xen/sched: Remove d->is_pinned

The is_pinned field is rather odd.  It can only be activated with the
"dom0_vcpus_pin" command line option, and causes dom0 (or the late hwdom) to
have its vcpus identity pinned to pcpus.

Having dom0_vcpus_pin active disallows the use of vcpu_set_hard_affinity().
However, when a pcpu is offlined, or moved between cpupools, the affinity is
broken and reverts to cpumask_all.  This results in vcpus which are no longer
pinned, and cannot be adjusted.

A related bit of functionality is the is_pinned_vcpu() predicate.  This is
only used by x86 code, and permits the use of VCPUOP_get_physid and writeable
access to some extra MSRs.

The implementation however returns true for is_pinned (which will include
unpinned vcpus from the above scenario), *or* if the hard affinity mask only
has a single bit set (which is redundant with the intended effect of
is_pinned, but also includes other domains).

Rework the behaviour of "dom0_vcpus_pin" to only being an initial pinning
configuration, and permit full adjustment.  This allows the user to
reconfigure dom0 after the fact or fix up from the fallout of cpu hot unplug
and cpupool manipulation.

An unprivileged domain has no business using VCPUOP_get_physid, and shouldn't
be able to just because it happens to be pinned by admin choice.  All uses of
is_pinned_vcpu() should be restricted to the hardware domain, so rename it to
is_hwdom_pinned_vcpu() to avoid future misuse.

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