virDomainDeviceValidateAliasForHotplug: Use correct domain defintion

https://bugzilla.redhat.com/show_bug.cgi?id=1553075

For some weird reason this function is getting live and
persistent def for domain but then accesses vm->def and
vm->newDef directly. This is rather unsafe as we can be
accessing NULL pointer.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

src/conf/domain_conf.c

@@ -5679,11 +5679,11 @@ virDomainDeviceValidateAliasForHotplug(virDomainObjPtr vm,
         return -1;
 
     if (persDef &&
-        virDomainDeviceValidateAliasImpl(vm->def, dev) < 0)
+        virDomainDeviceValidateAliasImpl(persDef, dev) < 0)
         return -1;
 
     if (liveDef &&
-        virDomainDeviceValidateAliasImpl(vm->newDef, dev) < 0)
+        virDomainDeviceValidateAliasImpl(liveDef, dev) < 0)
         return -1;
 
     return 0;