hyperv: cpu hotplug fix with HyperV enabled

With Hyper-V enabled CPU hotplug stops working. The CPU appears
in device manager on Windows but does not appear in peformance
monitor and control panel.

The root of the problem is the following. Windows checks
HV_X64_CPU_DYNAMIC_PARTITIONING_AVAILABLE bit in CPUID. The
presence of this bit is enough to cure the situation.

The bit should be set when CPU hotplug is allowed for HyperV VM.
The check that hot_add_cpu callback is defined is enough from the
protocol point of view. Though this callback is defined almost
always thus there is no need to export that knowledge in the
other way.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Roman Kagan <rkagan@virtuozzo.com>
CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Richard Henderson <rth@twiddle.net>
CC: Eduardo Habkost <ehabkost@redhat.com>
CC: "Andreas Färber" <afaerber@suse.de>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
