qemu: Fix typo in qemuDomainModifyDeviceFlags

One of our latest commits fbe87126 introduced this nasty typo:
func(vmdef, ...); where func() dereference vmdef->ncontrollers,
and vmdef was initialized to NULL. This leaves us with unconditional
immediate segfault. It should be vm->def instead.

src/qemu/qemu_driver.c

@@ -5833,7 +5833,7 @@ qemuDomainModifyDeviceFlags(virDomainPtr dom, const char *xml,
     }
 
     if (flags & VIR_DOMAIN_AFFECT_CONFIG) {
-        if (virDomainDefCompatibleDevice(vmdef, dev) < 0)
+        if (virDomainDefCompatibleDevice(vm->def, dev) < 0)
             goto endjob;
 
         /* Make a copy for updated domain. */