Use VIR_ALLOC_VAR instead of VIR_ALLOC_N for creating virObject

The current way virObject instances are allocated using
VIR_ALLOC_N causes alignment warnings

util/virobject.c: In function 'virObjectNew':
util/virobject.c:195:11: error: cast increases required alignment of target type [-Werror=cast-align]

Changing to use VIR_ALLOC_VAR will avoid the need todo
the casts entirely.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

src/util/virobject.c

@@ -186,13 +186,13 @@ bool virClassIsDerivedFrom(virClassPtr klass,
 void *virObjectNew(virClassPtr klass)
 {
     virObjectPtr obj = NULL;
-    char *somebytes;
 
-    if (VIR_ALLOC_N(somebytes, klass->objectSize) < 0) {
+    if (VIR_ALLOC_VAR(obj,
+                      char,
+                      klass->objectSize - sizeof(virObject)) < 0) {
         virReportOOMError();
         return NULL;
     }
-    obj = (virObjectPtr)somebytes;
 
     obj->magic = klass->magic;
     obj->klass = klass;