virsh: Require --xpath for *dumpxml

Historically, the dumpxml command reject any unknown arguments,
for instance:

    virsh dumpxml fedora xxx

However, after v8.5.0-rc1~31 the second argument ('xxx') is
treated as an XPath, but it's not that clearly visible.
Therefore, require the --xpath switch, like this:

    virsh dumpxml fedora --xpath xxx

Yes, this breaks already released virsh, but I think we can argue
that the pool of users of this particular function is very small.
We also document the argument being mandatory:

   dumpxml [--inactive] [--security-info] [--update-cpu] [--migratable]
           [--xpath EXPRESSION] [--wrap] domain

The sooner we do this change, the better.

The same applies for other *dumpxml functions (net-dumpxml,
pool-dumpxml, vol-dumpxl to name a few).

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2103524
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>

tools/virsh-snapshot.c

@@ -1609,6 +1609,7 @@ static const vshCmdOptDef opts_snapshot_dumpxml[] = {
     },
     {.name = "xpath",
      .type = VSH_OT_STRING,
+     .flags = VSH_OFLAG_REQ_OPT,
      .completer = virshCompleteEmpty,
      .help = N_("xpath expression to filter the XML document")
     },