kvm tools: Improve 'lkvm sandbox' syntax

kvm tools: Improve 'lkvm sandbox' syntax

This patch adds the possibility to run commands using the default parameter
in 'lkvm sandbox'. For example, a user would now be able to run:

	'lkvm sandbox firefox'

And have a sandboxed firefox instance start in the guest.

For commands with parameters, we still need to use dashdash to pass the
parameters (similar to 'git bisect run'). For example:

	'lkvm sandbox -- ls -al /bin

Suggested-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>