image: check for errors running mkfs / virt-sandbox
image: check for errors running mkfs / virt-sandbox

When running the sandbox commands, use check_call() instead
of call() so that we detect errors running external commands.

Signed-off-by: Daniel P. Berrange <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

agx/libvirt-sandbox-debian,libvirt/libvirt-sandbox,agx/libvirt-sandbox-debian,agx/libvirt-sandbox-debian,libvirt/libvirt-sandbox,libvirt/libvirt-sandbox,agx/libvirt-sandbox-debian,agx/libvirt-sandbox-debian