qemu: Fix ATTRIBUTE_NONNULL for qemuMonitorAddObject

Commit id fac0dacd was trying to make things more robust;
however, the ATTRIBUTE_NONNULL(1) would be for the @mon,
not the intended (2) and the @props argument as described
in the commit message.

Found by Coverity build.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>

src/qemu/qemu_monitor.h

@@ -824,7 +824,7 @@ int qemuMonitorCreateObjectProps(virJSONValuePtr *propsret,
 int qemuMonitorAddObject(qemuMonitorPtr mon,
                          virJSONValuePtr *props,
                          char **alias)
-    ATTRIBUTE_NONNULL(1);
+    ATTRIBUTE_NONNULL(2);
 
 int qemuMonitorDelObject(qemuMonitorPtr mon,
                          const char *objalias);