cli: improve tab-completion for types with must expression

[troglobit]

For veth peers and bridge-port bridge references we refer to an if:interface-ref which is guarded with a must expression. Klish, or rather its sysrepo plugin, should validate the alternatives it gets from sysrepo by calling the libyang function to validate. Then prune each alternative from the list that doesn't pass the validation.

[mattiaswal]

Looked in to this, seems that this is checked only when you do validate, in klish-sysrepo-plugin:pline.c. It seems very possible with not that much work.