virtiofsd: add container-friendly -o sandbox=chroot option

virtiofsd cannot run in a container because CAP_SYS_ADMIN is required to
create namespaces.

Introduce a weaker sandbox mode that is sufficient in container
environments because the container runtime already sets up namespaces.
Use chroot to restrict path traversal to the shared directory.

virtiofsd loses the following:

1. Mount namespace. The process chroots to the shared directory but
   leaves the mounts in place. Seccomp rejects mount(2)/umount(2)
   syscalls.

2. Pid namespace. This should be fine because virtiofsd is the only
   process running in the container.

3. Network namespace. This should be fine because seccomp already
   rejects the connect(2) syscall, but an additional layer of security
   is lost. Container runtime-specific network security policies can be
   used drop network traffic (except for the vhost-user UNIX domain
   socket).

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20201008085534.16070-1-stefanha@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>