nixos/test: Fix escaping for copyFileFromHost

A long-time issue and one of the reasons I've never used that function
before. So let's remove that todo-comment and escape the contents
properly.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @edolstra

nixos/lib/test-driver/Machine.pm

@@ -607,7 +607,8 @@ sub waitForWindow {
 sub copyFileFromHost {
     my ($self, $from, $to) = @_;
     my $s = `cat $from` or die;
-    $self->mustSucceed("echo '$s' > $to"); # !!! escaping
+    $s =~ s/'/'\\''/g;
+    $self->mustSucceed("echo '$s' > $to");
 }