Also add some argument checks to the Varlink function to avoid
referencing nil pointers, and complement the API.md descriptions.
The varlink endpoint can be tested via varlink CLI:
$ varlink call -m unix:/run/podman/io.podman/io.podman.SearchImages \
'{"query": "ruby", "limit": 0, "tlsVerify": false, "filter": {}}'
Signed-off-by: Valentin Rothberg <rothberg@redhat.com>