nixos-run-vms: If there is only one VM, attach stdio to its serial console

This is useful for interactive testing (quicker than logging in via
SSH and more convenient than logging in via a virtual console).

lib/test-driver/Machine.pm

@@ -50,6 +50,7 @@ sub new {
         stateDir => "$tmpDir/vm-state-$name",
         monitor => undef,
         log => $args->{log},
+        redirectSerial => $args->{redirectSerial} // 1,
     };
 
     mkdir $self->{stateDir}, 0700;
@@ -117,10 +118,12 @@ sub start {
         close $serialP;
         close $monitorS;
         close $shellS;
+        if ($self->{redirectSerial}) {
             open NUL, "</dev/null" or die;
             dup2(fileno(NUL), fileno(STDIN));
             dup2(fileno($serialC), fileno(STDOUT));
             dup2(fileno($serialC), fileno(STDERR));
+        }
         $ENV{TMPDIR} = $self->{stateDir};
         $ENV{USE_TMPDIR} = 1;
         $ENV{QEMU_OPTS} =

lib/test-driver/test-driver.pl

@@ -50,7 +50,7 @@ my $context = "";
 
 sub createMachine {
     my ($args) = @_;
-    my $vm = Machine->new({%{$args}, log => $log});
+    my $vm = Machine->new({%{$args}, log => $log, redirectSerial => ($ENV{USE_SERIAL} // "0") ne "1"});
     $vms{$vm->name} = $vm;
     return $vm;
 }

lib/testing.nix

@@ -158,7 +158,8 @@ rec {
         wrapProgram $out/bin/nixos-run-vms \
           --add-flags "$vms" \
           --set tests '"startAll; joinAll;"' \
-          --set VLANS '"${toString vlans}"'
+          --set VLANS '"${toString vlans}"' \
+          ${lib.optionalString (builtins.length vms == 1) "--set USE_SERIAL 1"}
       ''; # "
 
     test = runTests driver;