distro_bootcmd: add VirtIO distro boot command

Add a boot command to distro boot to support disks connected over the
VirtIO bus. The boot command uses the shared block environment.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alexander Graf <agraf@suse.de>