vsh: Annotate 'required' and 'positional' arguments explicitly

Add 'positional' and 'required' fields to vshCmdOptDef, which will
explicitly track the two properties of arguments.

To ensure that we have proper coverage, add checks to
vshCmddefCheckInternals validating the state of the above flags by
infering it from existing data.

This conversion will allow us:
 - remove VSH_OT_DATA in favor of VSH_OT_STRING
 - use VSH_OT_INT when required both as positional and non-positional
 - properly annotate which VSH_OT_ARGV are positional and which are not
   (currently inferred by whether an previous positional option exists)

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

tools/virsh-snapshot.c

@@ -1568,6 +1568,8 @@ static const vshCmdOptDef opts_snapshot_dumpxml[] = {
     VIRSH_COMMON_OPT_DOMAIN_FULL(VIR_CONNECT_LIST_DOMAINS_HAS_SNAPSHOT),
     {.name = "snapshotname",
      .type = VSH_OT_DATA,
+     .positional = true,
+     .required = true,
      .flags = VSH_OFLAG_REQ,
      .help = N_("snapshot name"),
      .completer = virshSnapshotNameCompleter,