hw/arm/virt: Add default_bus_bypass_iommu machine option

hw/arm/virt: Add default_bus_bypass_iommu machine option

Add a default_bus_bypass_iommu machine option to enable/disable
bypass_iommu for default root bus. The option is disabled by
default and can be enabled with:
$QEMU -machine virt,iommu=smmuv3,default_bus_bypass_iommu=true

Signed-off-by: Xingang Wang <wangxingang5@huawei.com>
Message-Id: <1625748919-52456-4-git-send-email-wangxingang5@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>