As a preparation to expanding Hyper-V CPU features early, move
hyperv_vendor_id initialization to x86_cpu_realizefn(). Introduce
x86_cpu_hyperv_realize() to not not pollute x86_cpu_realizefn()
itself.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20201119103221.1665171-2-vkuznets@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>