runtime.bash: fix check for parameter files

We need to check if the file exists, not just if it is a non-empty string.
While an empty $path would have the unfortunate effect that "cat" would
read from stdin, that is not an issue as you can simply not do that.
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

scripts/runtime.bash

@@ -118,7 +118,7 @@ function run()
     for check_param in "${check[@]}"; do
         path=${check_param%%=*}
         value=${check_param#*=}
-        if [ "$path" ] && [ "$(cat $path)" != "$value" ]; then
+        if [ -f "$path" ] && [ "$(cat $path)" != "$value" ]; then
             print_result "SKIP" $testname "" "$path not equal to $value"
             return 2
         fi