i386: keep hyperv_vendor string up-to-date

i386: keep hyperv_vendor string up-to-date

When cpu->hyperv_vendor is not set manually we default to "Microsoft Hv"
and in 'hv_passthrough' mode we get the information from the host. This
information is stored in cpu->hyperv_vendor_id[] array but we don't update
cpu->hyperv_vendor string so e.g. QMP's query-cpu-model-expansion output
is incorrect.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20210422161130.652779-2-vkuznets@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>