cli: Ad-hoc command execution

Enable ad-hoc command execution for CLI. With the previous commit which
enabled talking to a different instance (13a7576), this makes it possible to
execute remote commands while still using one's tools and shell.

For example (the executable name will be changed in future commits):

```
export OSV_API=http://192.168.122.89:8000
./cli dmesg | grep "foobar"
```

Signed-off-by: Or Cohen <orc@fewbytes.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
cli: Properly parse string command line

For the interactive shell, split the line to arguments properly (treating
quotes properly). cli() now accepts a list or a string.

Signed-off-by: Or Cohen <orc@fewbytes.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
cli: Refactor function names for consisitency

Signed-off-by: Or Cohen <orc@fewbytes.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
cli: Improve ls command

Add -l, -r and -t flags.

Closes #488.

Signed-off-by: Or Cohen <orc@fewbytes.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
cli: Fix table format padding

Signed-off-by: Or Cohen <orc@fewbytes.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>