qvm-prefs: refuse negative 'qrexec_timeout' value

This should be done in QubesVm class (property setter). But since we're
going to rewrite both qvm-prefs and QubesVm, settle on smaller change
for now.

Fixes QubesOS/qubes-issues#1273

qvm-tools/qvm-prefs

@@ -461,6 +461,10 @@ def set_qrexec_timeout(vms, vm, args):
         print >> sys.stderr, "Missing timeout value (seconds)!"
         return False
 
+    if int(args[0]) < 0:
+        print >> sys.stderr, "Negative timeout value not allowed!"
+        return False
+
     vm.qrexec_timeout = int(args[0])
     return True