fix #5607: schema: say what a valid configuration ID looks like

The 'pve-configid' format is used for many IDs, for example the names
of resource mappings, but an invalid ID only got rejected with:

  invalid configuration ID 'gpu@node1'

which does not tell what is wrong with it, so users had to guess or
look up the regex in the code.

Explain the rules in the error message: an ID must start with a letter,
be at least two characters long and only contain letters, digits, '-'
and '_'. These are the same rules the tooltip of the 'ConfigId'
validation in the web UI lists. Add a test for the message.

Signed-off-by: Michal Fox <me@dualfroz.com>

test/format_test.pl

@@ -25,6 +25,14 @@ foreach my $id (@$invalid_configids) {
     is(PVE::JSONSchema::pve_verify_configid($id, $noerr), undef, 'invalid configid');
 }
 
+eval { PVE::JSONSchema::pve_verify_configid('gpu@node1') };
+is(
+    $@,
+    "invalid configuration ID 'gpu\@node1' - it must start with a letter, be at least two"
+        . " characters long and only contain letters, digits, '-' and '_'\n",
+    'invalid configid error explains the format',
+);
+
 # test some string rendering
 my $render_data = [
     ["timestamp", 0, undef, "1970-01-01 01:00:00"],