kvm tools: Make 'vm sandbox' more user-friendly

This patch changes 'vm sandbox' to automatically prefix a program path with
"/host" in the guest side making this, for example, work as expected:

  $ ./vm sandbox -- ~/trinity/trinity --mode=random --dangerous

Cc: Asias He <asias.hejun@gmail.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>