PXB is now supported on ARM, so let's compile for arm_virt machine.

Acked-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Jiahui Cen <cenjiahui@huawei.com>
Message-Id: <20210114100643.10617-7-cenjiahui@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>