Enable use of -Wold-style-definition compiler flag

A couple of functions were declared using the old style foo()
for no-parameters, instead of foo(void)

* src/xen/xen_hypervisor.c, tests/testutils.c: Replace () with (void)
  in some function declarations
* m4/virt-compile-warnings.m4: Enable -Wold-style-definition

tests/testutils.c

@@ -457,14 +457,14 @@ virTestGetFlag(const char *name) {
 }
 
 unsigned int
-virTestGetDebug() {
+virTestGetDebug(void) {
     if (testDebug == -1)
         testDebug = virTestGetFlag("VIR_TEST_DEBUG");
     return testDebug;
 }
 
 unsigned int
-virTestGetVerbose() {
+virTestGetVerbose(void) {
     if (testVerbose == -1)
         testVerbose = virTestGetFlag("VIR_TEST_VERBOSE");
     return testVerbose || virTestGetDebug();